There's a lot of different treadmills out there of varying degrees of quality. We bought one mainly so the wife would have a chance to get some exercise while looking after our little nippers. However we ended up spending a bit more (£1000) to get what we felt was a good one that would last us a while. I use it myself during winter and think it's a great piece of kit.
Your choice will really depend on what features you want in the treadmill and how often you plan on using it. What top speed you want, how high an incline level, the motor power, linkage to your heart rate monitor if that's important, does it need to fold up, what length/width of belt......
I spent a long time researching ours and based on lots of other good reviews went for the Horizon 507 - So far it's worked out very well, had it for over 3 years now and haven't had any complaints.