As a rebreather diver in training, I can say a few things are critical...the first thing my instructor did before even having me start my class was watch this video....he wanted to create a mindset and I can't thank him enough..we all as divers have a tendency of accepting deviance and its a large part of what gets people killed...this video ALL divers should watch and its called "Normalization of deviance"
Lessons everyone should remember if you are seriously considering rebreathers:
1. Observer scrubber guidelines...if they tell you don't pack the scrubber without diving it within 24hrs, observe the rule.
2. Do your predives in completion, if the predive calls for you to check mushroom valves in the BOV...check them every time.
3. Scrubber packing is important, fly by that and expect trouble, don't repack, don't rush...do it right
4. listen to your instrumentation, if there is the slightest problem, Bail out... like my instructor said, "when in doubt, bail out"
5. stage bottle, you have no business diving a rebreather without a bail out bottle.
6. master the art of closing the loop when you surface...remember you are dealing with a chemical scrubber that does not like water.
7. Match your gas to your dive plan...this forces you to have a dive plan!
There are a ton of other things Ive learned but, these are the salient points. Nothing wrong with rebreathers, I think they make better divers because they force you to check your equipment in completion, something most O/C divers get out of the habit of doing over time.