At RCExitement in Mass. we have more than half of our stock 12th drivers running the SMC lipo. I'm planning to buy a lipo this week. All the remaining drivers plan to switch within 2 weeks.
Most of the recent lipo converts stuck with the 17.5 motors because they already have them. They are not much slower than the NiMh except for a little extra punch down the back straight.
I bought a 13.5 to be ready for the switch from my brushed 27T and now realize that I should have talked to the other drivers about it first. What seemd to happen was that most of them had round cell packs that were falling off a bit from age. The same people have healthy 17.5 motors. Once the first lipos hit the track, change came quickly as most were more than happy to dump thier sad round cell packs, yet few want to retire reliable 17.5s to the junk drawer. When the switch happens at the club racing level, it happens fast.