AMEX Web site (2012 Version) is decent
The web site changed earlier this year, so some of the pre-2011 comments in this thread may apply to the older version of the web site, not the current one.
I find it to be a decent web site (and the previous one was good too.) I happen to like the tags and the automatic rules you can assigned. MR point information details are a little buried, but I can't get any details out of chase UR, just a total of bonus categories, leading to "Did that last item post and I did I get bonus points for it?" type of thoughts. The MR points posting seems to be a procedural function (1 month behind) rather than a web site issue, although I supposed they could add the expected points-to-post in the transaction detail to make it more clear what you'll get when you get it.
There's a lack of Flash, animation and other web bloat which is always a welcome plus.
All other things equal, I'm more inclined to pull out the AMEX over other cards, because of the website & tagging, reporting and transaction detail download features. Chase offers some analysis in Blueprint, but it's their rules, not mine, so when the merchant is not correctly categorized I can't 'fix it' on Chase.
Is it perfect? No, but no web site is. I can deal with the few things that aren't completely to my taste.