They're not very aggressive, but have a nasty venom that causes progressive tissue necrosis if left untreated. What makes them the most dangerous is that they like to hide in dark places like the back of drawers where you might reach in looking for something. That, and the fact that their bite doesn't hurt much at first, so people tend not to realize they've been bitten, or think it was maybe just a mosquito or fly, so they don't go for treatment right away. If treated reasonably soon, there are usually no major complications, but untreated ones can be cause for limb amputation.
Gripped 7D, gripped, full-spectrum modfied T1i (500D), SX50HS, A2E film body, Tamzooka (150-600), Tamron 90mm/2.8 VC (ver 2), Tamron 18-270 VC, Canon FD 100 f/4.0 macro, Canon 24-105 f/4L,Canon EF 200 f/2.8LII, Canon 85 f/1.8, Tamron Adaptall 2 90mmf/2.5 Macro, Tokina 11-16, Canon EX-430 flash, Vivitar DF-383 flash, Astro-Tech AT6RC and Celestron NexStar 102 GT telescopes, various other semi-crappy manual lenses and stuff.