I have an MPC1000 and had an SP 505 which was the model before
the SP 606.
I couldn't punt
the SP 505 fast enough. It was hard to use, sounded like the samples were all coated with something they shouldn't be coated with and was poorly built.
The MPC1000 on the other hand is a keeper despite some variances in the velocity sensitivity of the pads. Maybe they've got this problem fixed by now, but I got mine out of the first batch and if we hadn't been going on tour in 2 weeks I would've sent it back. As it is, I paid the $120 for the 128 meg memory upgrade and $60 for a high speed CF card, and the MPC and I are friends again. The MPC is also a capable pattern-based midi sequencer, so it can do more than just drums.
If you've got the money, I recommend the MPC.