I have some experience with HP/Compaq blade servers, but no daily operational use. At my old company I was partly responsible for evaluating the HP/Compaq blade servers (this was just after their transition) for our datacenter and I can't remember any details other than the fact that we stopped considering the blade servers almost right away (after a few months) because of hardware, quality/reliability and usability issues.
I sure hope they have evolved (blade servers in general. really) since then, which was a year or two ago. Instead of the blade server route, we decided to go with a few hundred more 1U servers along the lines of the ProLiant DL360's. We did have the space and prior experience though, so we were comfortable with that.
Try to find someone with experience dealing with IBM and maybe even Dell blade servers, I'd consider them first. I have never touched one but from the news and reviews, the IBM Blade Center looks pretty bad ass! And like you mentioned, I have loads of respect for IBM's R&D and engineering, never had many bad things to say about any IBM product except that they tend to be more expensive. The HPs (which are basically the old Compaq line) seemed to be engineered very well, but they did have some quality issues.
I've just read back and realized that I'm rambling here so take this for what it's worth!
EDITed for typos