Pros: Best bass/mids I've heard and true to life sound
Cons: Not cheap, picky of upstream gear and not forgiving or poor sources
Here's a link to a wiki I wrote about the differences between the LCD-2r.2 and LCD-3s that pretty much sum up my thoughts quite well.
I've had them now for more than 3 months and they still get 80% of my head time. My HD800s/T1s/Ed.8s/HF-2s are sadly not getting the time they rightly deserve since the LCD-3s arrived. I was also able to sell my HE-6s as the improvements in soundstaging and air in the LCD-3s have made the HE-6s redundant in my collection.
Just a few more points, they (LCD-3s) are in fact much pickier of upstream gear than the LCD-2s (as both Skylab and Currawong have pointed out). A good case in point was the LCD-2/Lyr combination. I really liked the pairing very much, but the LCD-3s were quite a bit less synergistic. If I only had the Lyr, I would have been disappointed. But luckily I did have a WA22 (with the right tube combination) fed by my W4S DAC-2 on hand as well and WOW...what a pairing! I can also vouch for the GS-1 as well with the LCD-3s as they are a great match (for a SS option). Funny, I'm finding the LCD-3s as difficult to drive synergisticly as the HD800s.
As well, some of my older rock and jazz recordings that sounded "nice" on my LCD-2 (r.1 and r.2), where significantly worse on my LCD-3s. I don't blame the LCD-3s for showing me what crappy music I was feeding them, but if your collection is substantially populated with poorer recordings and you want a headphone to make them sound better, I'd suggest you look elsewhere. The LCD-3s or HD800s will serve to reveal their flaws and take away from your enjoyment IMO.
Overall though, the LCD-3s are my favourite "go to" headphone three months later, long after the "new toy" luster has passed.