I've only used the Jessops Photochem stuff until now which is very easy to use and seems to tolerate small temperature changes.
I have some different stuff on order (I forget the name!) and can let you know how that turns out.
C41 really isn't that hard. Like Redneckdan says, it's just a chemistry experiment. Stick to the times & temps & you'll be OK. If you want to try experimenting just keep accurate notes you can refer back to.