The only minor issue I have is the cpu power required to run version 6, without crackles, but I would expect that. If you want more power from your software and you don't expand your hardware to match that new performance then you are just asking for trouble. I think the general feeling from most FL users is that some tweaking of your ASIO driver will be required but once you get it stable again then you are quids in.
I immediately splashed out the extra dosh to get Directwave sampler. It is the dogs bollocks and fills a massive hole in FL's armoury. It links in with a download manager to Samplefusion site where if you are registered you can download multisamples straight into DW. If you need more then just buy one of the online albums and they will be available to youin the manager.
FPC uses the download manager as well to pull down extra drum kits from Samplefusion. Combine this with all the midi files that come bundled with it (now organised in a better menu structure) and you have thousands of ready made drum loops at your disposal. Quantize/Randomize the midi and the varitions are endless.
The biggest boon for me is the ability to slice patterns in the playlist. This means that you can now import a midi file and slice out a few bars quickly across all the instruments. A god send.
The new effects in FL6 are superb. Multiband compressor, Graphic Equaliser,Delay2 and Reverb2 have made massive improvements on songs I've been unhappy with. FL Squeeze has brought new life to old sounds.
I don't have anything bad to say about it yet, if I do you will be the first to know.