I use a Dual 867 G4 and I picked up Logic 5.5 about three weeks ago. I was making beats in Reason 2.5 and I wanted to be able to put vocals over the beats. I also have turntables that I want to record sets from and apply effects to and you can't do this in Reason either. I also needed something to mix down the music down a little better than Reason so after doing a lot of research and comparing it to other software like Pro Tools and Cubase, I ended up getting the Logic Audi Big Box package. I got lucky because the associates at Guitar Center didn't know a lot about the product so I talked them into giving it to me for only $200 bucks and some extra RCA cables.
From what I know so far, you can use your MPC with Logic, but there is a huge learning curve. I read in another thread, Vitaminman said it wasn't a two or three day software, and I couldn't have said it better. I am stilll trying to get the basics down. You are gonna need some sort of interface to connect the MPC to the computer whether it be midi, audio, or both.
If you had some way to capture the audio coming out of your MPC on your computer, you can use logic to edit the beats into arrangements and apply various effects to the song you created. The if you have a way to catch vocals, you can out performances to the songs. If you wanna use it as a midi controller, you get into a whole new chapter...
What is it you want to do that your MPC can't do for you? Are you trying to record vocals...Burn tracks to a CD form your computer?