I have both the full size and compact M&P 45's and will compare them to my Dept. issued Glock (G22 40 S&W) as well as a Glock 21 I used to shoot freequently (my fathers).
Pros: The grip size compared to other double stack 45's is far better. It is slimmer than the Glock 21 and fits my hand (and most others) far better. With the small grip instert, the grip circumfrence is the same as the FS M&P 40/9 and about the same as my issue G22. The recoil impulse seems very soft compared to other 45's including the venerable 1911, but that is my personal opinion. It is much lighter than a full size all steel or even a light weight 1911 when fully loaded and in my experience is far more dependable and less maintenance intensive compared to a 1911. Parts are easy to come by, Brownells and Speed Shooters Specialties have most parts in stock.
Cons: Some say the trigger is not the best, and while it is not great, it is certainly good enough for a fighting gun. Holsters are not as easy to come by without a wait time as the M&P 45 family is not a popular as the M&P 9/40. Some cite the ten plus one round capacity as a con, as the G21 holds 13 plus one, but for me it is a non issue. I have seen several reports of the 45's bending their magazine springs, but have not experienced it myself. The mag spring issue is the only real problem I have read about the M&P 45 family.
I love both my 45's and will probably try to get a mid size to round out my collection. Buy with confidence knowing that S&W has great customer service.