I sold my 404 years ago. It was before the SX came out. Then I had the 606 for a long time, but didn't do much with it. Now that I have the 555 I'm inspired to work with SP's again. I think it's mainly due to the LOOP CAPTURE feature. It makes it easy to do loops without using the sequencer. So that's really convenient for me. Also some of the effects are way more useful. Especially pitch is now a usable effect, which I think is a great move!
At the moment I'm looking into buying a portable 8track recorder. I think if you have something like that you would be good with pretty much any SP basically. Because I don't wanna get too technical anymore with sequencers and all. If I'd want to have control I'd use my Maschine. But I don't
Used to have a 404 back in the day and I think you were able to resample multiple pads onto one pad. For example playing drums and record them to a single pad. With the 555 it seems you're unable to do that, only through loop capture but then I'm limited to 15 seconds which is pretty short. Am I missing something here? 
