Well like a lot of things the answer is it depends; how many are you going to do, are you going to do multi-aperture, v grooves, etc, etc? The hand held jobbies might be ok for the occasional mat but you'll need some serious patience to lay it all out accurately and avoid overcuts.
Logan do a fairly cheap mat cutter which is ok but for a more professional tool have a look at Keencut; they're very expensive new but are always coming up on fleabay.
As already stated, it depends on what you want to do. I purchased a Logan Model 301-S Compact Mat Cutter (or an identical model) about 25 years ago. It doesn't get a lot of use but I wouldn't be without it. With a little care it can produce excellent results.
Like Jake, I'm not very impressed with the Jakar cutter I bought last year. Without checking I can't remember the model but I can use it freehand or clipped on to the aluminium ruler.
It's a cheap plastic blade holder not accurate, difficult to use and flexes like hell. Though I can cut a better bevel with it than using a scalpel, it isn't much of an improvement.
Waste of money, I should have bought a better one.