I use Firefox mainly due to lingering loyalty and ease. Loyalty- I've used Firefox for years, and when I first found it it was unquestionably better than the only real alternative, IE6. (I couldn't get on with Opera and the myriad other browsers all had their faults.)
However Firefox has been getting slower and more bloated with each release. Along came Chrome, and I like it. I do, it reminds me of when I first tried Firefox - something new, slick and fast. But I just can't make the switch because as well as loyalty to Firefox, Chrome for me is hampered by the lack of a linux version that I could use at home, and the lack of a portable version that I can run from a pen drive at work.
IE8 seems good, at least on W7 (I've found it a bit unstable under Vista). But still for me not as good as Chrome or Firefox.