I have the 50mm f/1.8 and the 100mm f/2.0 both are amazing. The fifty is best when stopped down though. I rarely shoot more wide open than 2.8. THe 100mm and its practical equivalent 85 f/1.8 are both sharp wide open and even more AMAZING when stopped down. LOVE my 100mm f/2.0. Cannot recommend it enough. In terms of focus hunting, if you have a speedlight you can always use the af-assist beam as one option even though it is not always appropriate to use it. Unfortunately I have no experience with the 17-55.