I didn't do a GTS course but I have had the priviledge of experience instruction from Dave Webb and Roger Henshaw when they were at PPSC. I also used GTS's notes to supplement the notes from my full time groundschool course (which was pretty good, but had some gaps: which GTS's notes more than adequately filled).
Basically, the GTS notes are brilliant. They are extremely clearly laid out; not too wordy (except where they need to be); very factual and the diagrams are very clear too.
All this backed up by probably about 100 years' aggregate (maybe more) of aviation experience from guys who really, really know how to put their subjects across.
It is also
"interesting to note" 
that Dave Webb is (or certainly used to be) a CAA consultant, writing some of the questions for JAA exams.