They're characatures of the geekiest aspects of scientists. Its like you got 40 real physicists and distilled out all the geekiness and put it in 4 people to the exclusion of all other character traits. Theres enough geekiness in me and I've seen enough in my mates and collegues for me to find it very funny indeed.
+1, the big bang theory is great. they're a bit 2-dimensional (especially wollowitz), but i think it's getting a bit better. and as you say, they're caricatures.
EDIT: oh, what have i been watching recently... lost just finished, 24 is nearly finished, dexter is pretty good, the inbetweeners is awesome :lol: i also got hooked on the practice, but its run seems to have finished too. big shots is actually pretty good, it's on RTE on a sunday night late. I think it got cancelled after one series, but most good things get cancelled to be replaced by an 11 series run of guff like will and grace o_O Oh and i watched the knights of prosperity which was pretty good too, but again its run seems to have finished. again cancelled after one series.