I switched at about 150 open-circuit dives. At that point, I had started working with doubles and taken some baby steps in the techie direction (adv nitrox, deco procedures).
I had been reading about rebreathers for a year or two, thought I had a good idea of the tradeoffs (gas management, risk, maint expense).
I decided that going CCR would be better for me in the long run based on the kinds of dives I want to do (50m plus) because:
1) I was pretty sure I wanted to do longer range dives, so why spend several years nickle-and-diming up a small fortune in techie oc-gear and then buy a rebreather and hope I can sell of of the oc-gear, or just pull the trigger in one big bang (e.g pay twice vs. just once).
2) I wanted the lower bends-risk from the whole "optimal mix" thing.
3) I wouldn't have to buy an SUV to haul my dive gear around.
I'm pretty happy with my choices, and at some point I expect to pretty much be diving mostly heliox. That is cool .
Some days I wish I had started from the beginning on ccr - I didn't expect the ccr learning curve to be as long as it is. I respect it and all, I'm just surprised how much focus it takes (compared to the simplicity of OC) for me to perform at a level I'm satisfied with.
So anyway, that was my motivation. I'm happy today. Ask me again in 3 years if I still think I made a wise choice.
John G.