I used a GPS for about 2 years, but deciding to stop using and just pawned it off on eBay for several reasons:
1. Having my pace in front of me at all times caused me to be overly competitive with myself. I would always end up pushing the pace in easy runs because I'd hate to see a slow pace on the watch. Maybe you're not this way, but a lot of runners are. Many of us tend not to have the best self control with training, so think hard about whether you do.
2. Trail pace is often times pretty irrelevant, especially on technical, steep, or undulating courses. Running an amount of time at a perceived effort level is far more important than running an amount of time at a set pace.
3. GPS are not always 100% accurate on trails, especially with lots of trees. If you know yourself well, your best estimate of distance is probably about as accurate as GPS on a densely wooded trail.
4. You will absolutely become dependent on a watch with this sort of data, which is not a good thing. Simpler is usually better.
5. If you want to get better, just go out and run.
If I haven't convinced you otherwise, just get a Garmin 305. They're every bit as accurate as the fancier versions, much cheaper, and the battery life is really solid (a big plus compared to the others, because let's be honest... you won't always remember to charge between runs).