I bought one a few weeks ago (Garmin 305) and I would gladly pay a lot more than I did due to how much encouragement it now gives me. Really helps me analyse my runs and therefore concentrate on what I need to do.
Flip side of it is it allows me to just switch it on and go run wherever I want rather than having to plan the runs in advance to ensure mileage - think it might even be helping me run slightly further than I would without it.
At the end of the day though it's a personal thing - where about are you? perhaps another forum member close to you might let you borrow it for a run to try it out???
I honestly dont know how I managed without it! It allows me to download training schedules to it - like interval sessions, beeping at me to speed up or slow down. I can then check everything online afterwards - and it syncs with GMaps so you can see where you were and what pace you were doing, so if you were slowing down, you can see you were going up a huge hill(!) Its a great motivator... you can see your best pace/average pace - for all of your splits. Its really easy to use as well(!) There is a setting where you can set your ideal pace... and it appears as a little running man, and your pace is measured against this (another little man) so ideally you want to keep up with the 'pacer' little man on the watch. Overall - there are probably a million other things it does that haven't been discovered yet!
pretty much at anytime... you can adjust how sensitive it is too, so whether it detects sudden changes in pace or 'smooths it out'. Average pace is also an option to be displayed. Funnily enough, nobody has mentioned yet that the 305 model has a heart rate monitor as well... all the info can be plotted on graphs to see what happened as the run progressed, or information from splits can be viewed in table format.
Obviously the slight drawback being it won't work with a treadmill since you're not physically moving anywhere.
As ed says, you don't need one (I mean you don't really even need a stopwatch) but it gives you all the information you could possibly want to monitor your training progress.