Nokia 5800Please do forgive the long post, hope it's useful!
I've had the 5800 for a while now. Overall, it's a nice and capable mobile phone. I won't go into detail, but some of the usual worries people have voiced about it are the relatively unsensitive touchscreen (compared to iPhone) and the scarcity of available third party software. Also, if you are used to the Mac OS X interface, you might find the GUI and synching with your Apple a bit unfamiliar, compared with the iPhone. These issues are discussed in detail elsewhere. I'm very much satisfied with its phone functions and would gladly recommend it, especially in its price range.
The sound quality is nothing to rave about, really. It's all adequate for everyday commuting though. I've tested it with good quality MP3's and several phones ranging from Koss Portapro at the lower end, to Grado SR80 at the upper. Honestly, i don't think it's much worth using better headphones than that, Grado SR60 is about what it can handle, IMHO. I never ventured into the lossless formats with this phone, didn't think it to be worth the effort. It does a bunch of formats like AAC, WAV, MP3, MP4, WMA, etc., and a host of other less usefull ones as well (please see documentation for more detail). No FLAC or OGG though, as far as I know. Might be worth to give WAV and AAC a try after all, especially for certain styles of music; though i didn't feel that there were any format related problems besides the usual MP3 crapiness as expected.
It's a bit similar to my first generation iPod Shuffle (though worse, TBH) – good channel separation, nice stereo width, OK separation throughout the freqency range. It has a certain dynamic, lively quality, although not very much of it. No real depth in the soundstage, but with the separation and the width it's not really disturbing, there's enough air for the music. Tad unnatural with properly recorded classical music and the like. Also some muddy lower mids and kinda' shrill upper mids every now and then (depending on headphones and source music), but again, i don't think it's a big problem, considering it's a phone. The bass is okay, even rather bass-heavy electronic music comes through solid enough. No big bassline pressure though, if that's your cup of tea, you'd need to use either the EQ (not good) or possibly an amp (didn't check, might work). Treble is good, enough detail to have an enjoyable level of musical subtlety.
The EQ is very unnatural, not really useful in my opinion. The volume control can be a pain, it steps too steeply, rather hard to find just the right level without some tactile fiddling with the volume button. Do stay away from the earbuds supplied with the phone, they suck so much i have no words to describe it. I have a certain impression that the headset, that has a normal 3,5 mm jack for headphones, degrades the sound quality noticeably (that is, even if you plug good headphones into them) – this is just theory though, i nevery really checked thoroughly.
I do not use any amps with the 5800, again it seems meaningless in the context. Also, i'm skeptical of the double-amping that occurs; amping the crappy colouring of a bad built-in headphone amp, idunno... It might be worth a try though, i can definitely see the Fiio e5 giving just the right push, with the bass boost, maybe some clarity, etc.
The music player software works nicely. Simple stuff really, the usual display of basic track info, cover art, etc. Transferring your music from computer is easy, whether through the supplied software (didn't try, don't like the idea, said to work good though) or through using your phone as a USB storage device (worked very well). As with most other phones, the music player is rather heavy on the battery! Not catastrophally, but yes, it does make a tangible difference battery life-wise. The phone uses a standard Micro SD memory card.
Compared to my Sony-Ericsson w810i, it's a much "better", more versatile phone, with a very different, all together more boring sound signature. The w810i had some aggresive dynamic quality, which was in a way more fun to listen to, given the right headphones, despite it's strong colouring and general harshness.
To wrap it up: If you are going to use it for commuting, casual listening and general entertainment, it works just fine. Nice to be able to listen to music in decent quality, surf the net, check out your schedule, put down some notes, etc., all with the same unit. For a bit more serious listening, i strongly recommend other choices!