Yes I have one, I sent my MPC 2500 back to Musicians Friend after it took me 2 hours to chop this song (you can hear it here myspace.com/tharemarkablestone). As you will hear it only has drums that are on the record, i was so frustrated after chopping for and getting it to sync I sent it back. Now I am not telling you to get one, I dont advocate for one machine over another, but I was spending more time on MPC's editing and trying to sync that I was not making music. So as far as features, I think if you are a loop base producer and you want to sync a lot of loops together with ease I like the MV. Its so easy to change patterns or and insert different loops. Like If you have 10 patterns or you can have up to 99, you can use any of them no matter the BPM, you give the number of beats, turn BPM sync on and change patterns on the fly and they will sync. Editng features are one click away, not menu after menu, check out madman at
http://www.phillyheat.com/ and he does some real world applications as far as sampleing and stretching write here
http://www.phillyheat.com/user/Mv4b_ep2Part2.ram and if you want all the videos he does see here
http://www.phillyheat.com/documents/8000.php
All machines have their weakness, It was hard for me to understand patches and library because thats how the MV stores samples so you can have them all the time. I dont know what will work for you, and I never worked with a MPC 4000. THe MV has 8 audio tracks, so you can import acapellas that will sync with the 136 midi tracks or record in live instruments, it ships with 12 of stereo sampling time, (128mb) exapandable to 512, I did not expand as that is plenty of time for me for now. On the MPC 2500 I would only sample 2 bars to chop because after that it was hard for me to sync, now I sample like 6 to 8 bars, it has a quick edit for fast pad assignment, it will hold 512 audio loops and there a feature to trigger them live into the sequencer or sync them up. Check out the vids, hope this helps, as I just got mine last week, I will be posting music on my space next month, I will post it here somewhere when I do. Again, I dont advocate machines over others, maybe you may need a software program to make it easier for you or somthing simple than giving up your MPC just to get a MV and find out you hate it.