I bought a treadmill late last year SE and I am so pleased with it.
As Steve said for a half decent one you do need to spend a few quid.
There are many factors to consider aswel.
I did read a lot of reviews and think it would be worth your while researching before you buy as it would be a lot of money to spend only to find that it's not up to scratch.
The main things that I required were the warranty.Some don't offer as long as others the average seems to be about 1 year parts and labour.
I managed to get 3 years warranty with a 10 yr motor warranty.
The top speed on most treadmills up to about £1000 is about 10 mph I wanted a faster one that that,and mine goes up to 12mph,it also has the other options such as incline,hill work,intervals etc. The treadmill has improved my ability to run at a more constant pace on the road.
The belt width varies aswel,the norm is 18" but you can get some with 20".The extra 2" doesn't sound much but does make a big difference.
If you have room for a fixed then that is probably better(according to what I've read)but they are usually dearer.Mine is a folding type.
The frame work is more reliable if it is welded rather than held together by nuts and bolts as over time they will loosen.
if you look at treadmill doctor they give ratings and comparisons for most price catagories.
My one is a "smooth fitness" make and I can't speak highly enough of it and the service was great.Incidentally,the belt on this model is orthapaedic apparantly,so even kinder on the joints than the average treadmill in it's price range.Don't know how they do that though.
Didn't intend to bore you with an essay!:eek: