CO2 lasts for ages. I picked up two 6kg fire extinguishers and can't see me needing to source gas for the next couple of years at least. If you're lucky, your work might end up needing to replace some extinguishers...
So even if you have to buy your gas, it will last you ages.
Pros:
Crystal clear sediment free beer. Assuming your got the rest of the brewing process sound too, of course.
Quicker than bottling, by about 2 hours which is a boost.
Quicker turnaround (carbonation), although not by much if you're after the clarity.
Cons:
Requires a dedicated kegerator/keezer which ultimately takes up space. This isn't ideal for anyone really.
Initial setup cost can be expensive. Even on the cheap you're looking at over £150 for a two corny setup.
If you have the space it's absolutely worth doing, I have a fairly ghetto kegerator in the garage built from a £25 second hand fridge freezer. Does the job and I'm happy enough.
