I'm a lefty...
...and have repeatedly distressed my disappointment that Springfield and Kimber consider a saftey I can use a custom feature, whereas it's standard on many low end clones.
Anyway, about slide releases. Slide releases are traditionally on the left side of the frame, right? Okay, unless most people have thumbs a lot longer than me, there's no way you could reach that with a firing grip. I, however, can reach it with my left index finger easily, just as I can easily reach the mag release with said finger.
If the gun's only saftey is a decocker, ala Sig P226, I see no point in ambi controls. I can't imagine having to do a "combat decock" under stress, so it's a moot point, as far as I'm concerned.
Guns I think are good for lefties:
1911s with ambi safties-
Springfield XP series-
Glocks-
Rugers-
CZ 85-
Beretta 92/96-
Taurus PT92/100-
...And just about any double action only auto. No saftey, no problem.