Hi,
We have had Go4Schools for 12 months now and run it in conjunction with SIMS. We've striped SIMS back to bare minimum functionality and operate the following systems using GO: Markbooks, Data Analysis, Behaviour and Attendance. As you quite rightly point out Seb1780 the Behaviour side of GO needs some development to catch up with SIMS but we've been happy with the functionality of all the other modules.
Detentions is something GO have been developing since we brought the system off them and I have to admit there has been very little change to the Behaviour Module. I hope this is something they look at over the next couple of months.
If you want to catch about our experience, just let me know.