So I didn't have much time, after work, to play around with the 300mm/1.4x Teleconverter combo, but I was very impressed in the short time I did spend with it. It's not terribly fast, the IS isn't great and it doesn't focus as quick as my 17-55mm, but God damn is it sharp. This was taken with the 1.4x teleconverter attached - I was expecting to lose quite a bit of quality from it, but - while I did lose some - it is still amazingly sharp. Much more so than my 17-55mm.