That's a pretty good deal, but I don't know how useful it would be to have 50 of that size. My two most common sizes are #71 and 1mm (about #60). The #71 is good for small component leads (most resistors and capacitors, TO92 transistor legs, LEDs, etc) and the 1mm is good for most larger stuff (TO220 power transistor legs, pretty much any IC socket legs, 0.1" square pin headers, etc). I bought a box of 50 of the #71's, but it was worthwhile because they really are fragile. I've managed to break probably 10 of them in a couple years, but since getting a real proper dremel drill press setup, it's always my own mistakes that break them lately, not the drill's fault. I pretty much never break any of the larger bits, such as the 1mm. Those #66's you linked are right about in the middle of my two favorite sizes - they'd probably be a bit large for the smaller stuff that you may want to squeeze close together, and a bit small for the larger stuff - plus, they'd probably be a lot less fragile than the #71's - not a bad size to have around but I don't think I'd want nearly that many of them...
My $0.02...