MPC's main advantage is being an all in one box for beatmaking.
You get integrated sampling, sample editing, and midi sequencing.
It has a midi sequencer that is pretty much designed for electronic style music such as Rap.
I still find the Mpc to be the most comfortable workstation made.
IMO Fruityloops just don't have the real time response as a mpc. It doesn't seem suited to low latency playing.
Reason, Live, Sonar on the other hand just doesn't have the feel of an MPC. Even with the Kontakt, MPC pads (i don't even use mine), Sound Forge, Recycle etc.
I hear Cubase is about as close as it gets (although Live does the pattern chaining function even better).
Certain MPC features to look for in a sequencer.
Auto Quantize (Every soft sequencer has this about now, but Project 5 doesn't right now. it will get this shortly).
Metronome with configurable time rates (Cubase has this)
Note repeat (Most sequencers have arpegiattors but are more difficult to use).
Dedicated Overwrite and Overdub functions (most software seqs use a switch to accomplish this. I find the buttons more instant access.)
Unique Overwrite loop record mode which turns into Overdub when loop point is reached (Only a mpc user will know what I am talking about. The advantage is not having to press undo for a bad take. This is what really make the mpc quick to compose).
Not having to arm midi tracks to record (ableton live and Project 5 fail at this)
Being able to record new midi tracks while sequence is playing (older mpcs couldn't do this fully. Live, P5 does this good ).
Sequence chaining (Ableton Live, Project 5, and Cubase does this. Keep in mind Live and P5's way of doing this involves more time setting individual lengths of each track. Mpc doesn't have this problem).
Built-in sampling and editing (this can be done on most computer sequencers. It is just not as pretty. Reason can't do this at all).
Loop/ Tempo calculator with pitch shift adjustments( Sonar has this to a degree (but it doesn't calculate new tempo when speeding up loop).)
Works with external midi devices (Reason can't do this)
Using Pads to mute tracks in real-time. PC users are typically stuck with a mouse unless they have a controller.
When you find something that does all of this on the computer, post back. The computer kills the mpc though when it comes to actually having to edit midi (mpcs are horrible at this), step writing midi, recording vocals, and mixing your song. Also you get a better feel with at least one external midi controller/synth with a mpc like a roland fantom or triton.
Another advantage (on some mpcs) is the sounds of the samples themselves. the 3000, and 60 are some hard hitting machines, but to be fair you can get this (and probably better) effect using plug-ins, hardware samplers like the s950, s3000, ASR 10, sp 1200.