Modern compact flourescents seem pretty good about getting to full brightness fast. The 100w and 60w (equivalent) spiral ones I've bought this year seem to be pretty much instant on.
But I also bought a bunch of 65w equivalent spotlights for recessed lighting and put those in for all 5 of the recessed lights in my kitchen. I noticed when I did one, that it was a little slow to reach full brightness, but with all five, it's like they're all slower again. Is there any reason multiple CFLs on the same circuit would be slower, or is it just that the encased, recessed CFLs aren't as good as the simple spiral CFLs?