Being a big Roland fan and also a deejay, I like the 909. The thing that seperates it from the rest is that roland quit making the 808's (their biggest sampler) and incorporated it and most of it's most popular features into their 909 groovebox. They reintroduced the 505 to take care of those who wanted a semi-powerful sampler. Eventhough I'm looking at this from the point of being a deejay, It has some awesome features for producers. Bottom line, it all boils down to how you plan on using it. In my opinion, the 7000, mpc and 909 all have different strong points but for getting more variety and versatility, the 909 stands alone.
--dac