If you've used a Dremel before, you know it's a great, handy, reliable tool for small household work. Unfortunately, the 400 line is only two of those things: after only moderate use, my motor died. This has not been an uncommon complaint with the latest Dremels, as people on Dremel's official forum loudly proclaim.
I had only put in around 5 hours cutting mostly particle board and PVC before the motor began to smoke. In its last hour of use, I'd use it for a few minutes then let it cool down completely before trying again. Everything was done in a well-ventilated area in sockets that didn't show signs of running too much current.
It's unfortunate that if it weren't for reliability issues, it'd be a great product. The XBR comes with a Flexlite, a circle cutter, and the bit extension, plus around 100 useful bits. The case is nicer than the 300 series', and the thing looks a little neater too.
Get one from the 300 series, avoid the 400 XBR.