I've never used them myself, but the buggy guys at my track have mixed veiws. Torsen VS Spider



found a couple of links to some VERY informative pages about Torsen Diffs & Varilock Diffs & their operating principles!
this should be some help to those of you interested in either buying a Torsen or trying to decide between a Torsen or one of the new Varilock type diffs soon to be released by Fioroni the Tractive IV



good bit of reading there if you're feeling technical


Torsen theory & applications inc downloads


Varilock theory & construction

cutaway view of diff internals Tractive IV

hope this is some help :C: