The most serious problems with all of the IE family (and other "internet aware" products from M$, like Outlook) is their very poor security track record. You really need to keep up-to-date on installing the security hotfixes/patches/service packs if you want your surfing to be safe. But the service packes themselves have a history of breaking things, which is why, once burned, people tend to avoid installing them -- as well as the sheer irritation of havingto do it...
As for useability/whatever, well, you'll be using a product that a large proportion of website designers
assume that you will be using -- so much for software independence, as I remarked on
another thread...