Pros: Incredibly comfortable, Great sound, Huge sound-stage for inexpensive closed cans, no need for amp (though it certainly helps).
Cons: Somewhat fragile (not a problem with proper care), Non-portable, Bass might be too light for some users, just OK sound isolation.
With that out of the way, lets get to what makes these great.
They are incredibly comfortable - so much so that I can wear them literally all day long without any discomfort. They are very light weight, do not apply much pressure to the head, and despite being closed cans, they do not get particularly warm on the ears. The super long, high quality braided cord gives plenty of freedom of movement (and can take quite a punishing from chair wheels). An added bonus is that the "3D-Wing" support system, in addition to being super comfortable, is also an automatic-adjustment system. This means you never need to manually adjust the length of the headphones. The only negative thing I can say regarding the comfort of these cans is that users with long hair may occasionally snag their hair on the various moving parts, that said, I have quite long hair and have never had any real issues with it.
They sound great, especially for the fairly low price you can frequently find them for on Amazon.com. They do not require an amp, although using one certainly does make it even better. The medium sound isolation strikes a good balance between keeping unwanted outside noise at bay and still allowing "needed" sounds to be heard (a knock at the door or a phone call for instance). They have a wonderfully expansive soundstage for closed cans and are excellent for music and gaming alike. The mids and high are crisp and pronounced, and while the bass is certainly not the powerful, it is very clean and pleasing. As long as you aren't looking for particularly powerful bass, the sound on these cans are without doubt worth the price. For anyone who hasn't used similar or greater quality cans before, you truly need to hear these to appreciate them over your average headphones.
Now, there is one fairly serious weak point to these cans (aside from the lack of portability). They are somewhat fragile - perhaps too fragile for most users. Mine lasted two years before another family member dropped them and damaged one of the drivers causing it to rattle at certain frequencies. After sending it in for repairs, it lasted another year before developing the same problem once again (without being dropped again as far as I know). While this isn't necessarly a poor lifetime for headphones of this price range, it shoud also be taken into consideration that I am extremely careful with my computer-related items and my experience is likely more than most users can reasonably expect. That said, if drops can be avoided, I expect these to last a very long time as the cable and other "common" breaking points for headphones are all of very high quality on these particular units.
As of writing this review, the A700 (and A900) have been discontinued and replaced by the A700x (and A900x). Some sellers such as amazon are still selling the older versions (significantly cheaper than the new "X" models) but that will only last so long. The X models are significantly more expensive than their older models (Same MSRP, but the "X" models are selling close to MSRP while the older versions were often over 50% off), although they do come with a few improvements. If considering an "X" model, consider it a small upgrade in every respect aside from perhaps the durability of the "3D-wings" which feel a bit cheaper constructed, although in use I haven't noticed any difference (in fact they seem to perform better).
It's also worth noting that Amazon and other sellers now offer extended warranty plans such as SquareTrade, which can substantially improve the durability issue for a reasonably low price.