Kottke explains the problems with Technorati (via, and more from, Neil McIntosh).
Technorati is certainly pretty rough around the edges, but I still think it’s the best out there for what it does, which is basically just getting bloggers all pleased with themselves when somebody links to them.
I use a mixture of Technorati, TTLB Ecosystem, and more recently BlogPulse. Nothing out there seems particularly reliable to me (although I like all the graphs and stuff you get on BlogPulse).