I don't that the pre-reqs for TDI rebreather courses are particularly high - you do need an Advanced Nitrox cert (up to 100%) but that training can be included in the RB course. Otherwise, I think it's AOW (Rescue preferred?) and a level of diving experience.
The thing that Pete and every other RB diver I know keeps re-iterating is this.... you might be an experienced diver on open circuit, but you'll be a noobie on the RB for a long time.
I've been thinking about it for a while, and I do tend to flip-flop a bit. I haven't found my "perfect unit" yet, and I've also realised that 70% of my diving is with someone who doesn't want to dive closed circuit yet. So the advantages of a RB won't be really utilised.
I'd also suggest talking to a few RB divers about the "hidden costs".... sorb, batteries etc. They aren't cheap to buy, and they're certainly not cheap to run.
I'll probably commit to a RB when one of two things happen - when I want to extend dives to deeper than 55m in which case I can't afford OC trimix fills or if we move somewhere with cold water, as breathing compressed air is one one of the main factors for being cold on a dive. Doing long deco times after a wreck dive in Canada does not sound like my idea of fun if on OC in cold water....... brrrrrrrr!!!