I'm a lefty.
I think they are pretty ambi. You have to get used to hitting the mag release with your trigger or middle finger. This is no biggie if you ask me as I shoot IDPA and do not feel slowed by this. In fact some righty's complain about the mag release for them because if you grip the gun a certain way your palm counters pressing the mag release with your thumb.
The extended slide release ($15) is a nice option (a friend has one), but I don't find the stock one offensive enough to have bothered yet; probably because I do the slingshot method of putting one in the pipe.
Let me qualify by saying it's the first and only handgun I own. Those trained on other guns might not adapt as happily to the Glock as I have.
Glock 'revised' safety rules

1) Keep your finger off the trigger.
2) Keep your GD finger off the trigger.
3) Keep your finger off the GD trigger.
4) Keep your GD finger off the GD trigger.
Basically, a good holster is the 'manual' safety for a Glock.