Pros: Balanced but musical, excellent isolation, lets good recordings shine, amazing customer service from ACS - still my go-to for 2+hrs a day
Cons: Cost, need tender loving care, isolation worsens conducted noise through your skull, can sound congested, sibilant recordings may become unlistenable
DOI: I don't get to listen to new gear a lot so I may not be any use at answering questions about comparisons to anything other than my own stuff (which may or may not be considered esoteric). I was (and am) a full-size hi-fi person so my tastes in this will clearly colour what I look for in head-fi. I grew up playing instruments and singing in a choir so I tend to value realism of tone for well-recorded voices and instruments but for orchestral works and pop/dance music, I look for neutral-warm musicality and dynamics.
There isn't a lot of choice for customs in the UK though things have improved over the 3 years I've had my T1s. My choice of ACS was mainly down to price and them being English with an excellent return policy. Overseas outfits would carry import and/or shipping-for-refit charges and my top-choice Spiral Ears probably wouldn't have fitted my small ear canals.
Monika Schumacher (www.protection4hearing.co.uk) was recommended through Head-Fi and she deserves special mention - an amazing home visit service for taking the moulds and arranging the manufacture with ACS. Most notably, after I'd mentioned pain/fit issues I'd had with custom silicone eartips for my Senn IE8s, she strongly suggested I avoid customs as I might be in for further disappointment. Thankfully, I ignored her
Please bear in mind that the standard cases and accessories for ACS products have changed over time. The T1-equivalent today is the ACS Evolve with slightly different electronics and a Linum Bax cable, though I believe you can request the original version as they now have their compatible Live! ambient processor for sale. They also now do universal versions.
Rigid, brown leatherette slide case for storing the 'phones, comfort cream, 1/4" adaptor, wax pick and wax-grid dispenser - too chunky so I used the matching zip-pouch for day-to-day carry.
The T-series cable is long but very strong and not very microphonic. It had a pronounced tendency to auto-coil but tidied up easily.
Fit and isolation:
I took some getting used to these. Fit was great as expected but coming from super-light Senn IE8s and Complys, I am clearly not designed to have things comfortably jammed in my ears. That being said, it got much better over time. The silicone is very flexible and getting them in and out is much easier once the lacquer becomes less sticky. The main issue for me is they need to warm up before they 'disappear' and obviously, YMMV. The excellent seal means that I need to wiggle them around and pop my ears (like when diving) so they sit without 'sucking' on my eardrums - otherwise the bass and soundstage can become lop-sided. The other downside to the isolation is that you notice the noise of blood rushing in your ears and your own footsteps much more readily (and I find this harder to ignore than ambient external noise).
iPhone 4 w/ ALAC initially. The BSG cmoy and my Pimeta v2 amps didn't get much use with the T1s as they didn't seem to play nicely together. Lately, I've been using an iPhone 5S with the Onkyo player app and an iBasso DX80 with Lurker's firmware (ALAC and Hi-Res FLAC).
A Few Thoughts on the Sound:
Not a bass monster but it's certainly faster than my IE8s and goes a lot lower with enough juice - it does a decent job with my favourite speaker torture track "Why So Serious?" from The Dark Knight OST which goes from screeching strings to barely audible but very palpable bass with a snare drum overlaid. As mentioned in threads passim, voices are a treat with these IEMs: the ladies from Girlyman's live album Somewhere Different Now are wonderfully euphonic in the higher registers, as is the soprano in the 1990 Bernius recording of JD Zelenka's Missa Dei Filii. Patricia Barber's dark contralto is wonderfully husky on Cafe Blue. Instrument tones are generally very good though a bit peaky in the upper frequencies - I certainly wouldn't accuse them of being over-smooth; cymbals are a touch sizzly, the crunchy-sounding guitars on Linkin Park's Hybrid Theory can grate whilst they slam. Processing artefacts like clipping and excessive 'loudness' such as (the entirety of) Florence + The Machine's Ceremonials stick out like a sore thumb and can be rapidly fatiguing.
Now comes the big but: sibilance. I have no idea if this is a quirk of the drivers' frequency response in the 'presence' range or something to do with my set-up but with the wrong recording, it can make me want to chew pennies for some light relief. Good live recordings, unsurprisingly, do better in this regard (compare the studio and bonus UK live tracks of Fun's Aim & Ignite) but even the aforementioned Cafe Blue has this in relative spades (it's certainly evident on my full-size system which is, if anything, rather bright but the distance in space to the listener mitigates this a fair bit) and I suspect is partly due to the miking.
I'd also suggest that these aren't going to replace full-sized phones for large, orchestral works. The soundstage is spacious though not as 'out-there' as my Senns - this is a positive for intimate tracks where the performers are sitting close together, like Somewhere Different Now or Ottmar Liebert & Luna Negra's Up Close (binaural recording!) and doesn't disgrace itself compared to full-sizers. When there are instruments clashing all over the place though, like Messiaen's Turangalîla-Symphonie, it can all sound rather congested, especially at volume.
Care & Service:
These need regular inspection, cleaning and drying to maintain their edge - the tiny, plastic wax-grids that fit down the sound tubes tend to break when removed and aren't cheap bearing in mind you need 4 at a time. Without regularly changing the silica gel or storing these in the airing cupboard, condensation can and will become evident around the components under the silicone and cause one to freak out! However, a careful clean has never failed to restore the sound for me. I have had to return these to ACS 3 times so far - what turned out to be the cable damaged internally at a particularly sharp bend over my right ear and 2 driver replacements (the second out of warranty but at a very reasonable cost of ~5% of what I paid initially + postage). Given that in the early days, these lived in a soft case in the top of my rucksack being bashed around, I'm not surprised. ACS have always been quick to reply, courteous and a joy to deal with. They also warranty their repairs for an extra year.
As mentioned, I've had to return these to ACS a few times and subtle changes have occurred over time: the sound tubes gained little in-line filters and new-design wax-grids. The trapezoidal leatherette case has been replaced by a more practical zipped semi-hard-shell after I lost the little pouch. My second cable failed at the right internal plug-end after just over a year but I was sent a (free!) replacement that was identical in feel but is much less self-coiling. Best of all, I now have a Revivo (included with their new customs) which is a USB-charged drying box with a replaceable silica pod and a UV radical generator to help sterilise the earpieces. It's a lovely idea and slightly more practical than a hearing aid dryer unless you have loads of IEMs.
The T1s have been my everyday listen for 3 years now. For commuting their isolation is fantastic, comfort (once warm) nearly unrivalled and their sound detailed, engaging and musical as long as you can tolerate their unforgiving and revealing nature.