One thing everyone should know who contemplates switching over to
R134: you will need some helping heat to get it all out of the can and
into your car's AC system when you go to fill.

You will find that unlike R12, R134 needs some help. When you connect
it and begin the filling process, everything will go fine until the
last quarter of a can or so, which will just sit there in the can.
Always have a pan of hot (boiled) water ready to stick the bottom of
the can into. It will cook off the last of the coolant. Watch the
pressure gauge on the meter (even the cheap refill adapters offer
them, it is worth looking for one) which normally has a scale on it to
take the ambient weather temperature into account.

I've never had a problem, but I wear eye protection.

Jim