My $0.02 as someone who played the game while it was beta and became fairly good, then only picked it up again a few months ago and is newbie at the many things that have changed, and is rusty to boot.
MP5 - probably the best all around. Excellent rate of fire, accurate, and pretty cheap. The best thing about it's accuracy is that it is pretty accurate in full auto too (a rare trait). Combine that with the technique common to all smgs of not standing still (if you are standing still, a rifle is always better), and it is probably my favorite weapon. If you can keep moving, however, and have a hiding spot, it is good for long range too, IMO, contrary to what others have said.
TMP - stealth is good, especially if you can come at someone from an unexpected direction. Other than that strength, that will vary in usefulness to you depending on your style of play, it is worse than the mp5 in every way. Well, the rate of fire is good, but since the full auto accuracy is much worse than mp5, this is negated somewhat...though, like the first few shots of an ak, you know exactly how the shots will grow innacurate (i.e., it travels upward) and you can compensate for it for a while.
Mac 10 - already stated...close up, this thing is DEADLY, medium range it sucks except with the 1 or 2 shot burst fire method that works for any automatic gun, and just forget about long range.
P90 - YECH to the full auto of this baby. It might be the worst full auto smg, not sure. If you can restrict yourself to 2 and 3 shot burst exclusively, however, it is a very deadly weapon. Especially handy is the 50 shot mag size and how long it lasts if you use it this way.
UMP - If I remember correctly, this is the VERY odd looking, almost triangular, gun. Slow rate of fire as has been said, but this allows it to be a bit more accurate full auto. If you're aim AND tracking (keeping them in your crosshair as they move quickly) of a target are good, or if they stand still for some reason, fairly effective. I have a lot of trouble tracking moving targets with this because of its rate of fire, though, atleast when I am moving too. This to my mind is a major weakness for a smg, so I never buy this gun.
Are there any other smgs? Can't remember any. Considering how cheap the mp5 is, I hardly buy anything else.