Okay, so after some tinkering with my tablet on GIMP I found out two things.
1. Short, quick brushstrokes work just fine with no pixelation. So if your art only needs that kind of paint action then you should be fine.
2. BUT if you happen to want to trace a scanned work with long lines across the screen then you're out of luck. You will get pixelation no matter what. If you want to trace something scanned that you've already drawn then I'd have to recommend vector lines to do it.
Just got a Wacom Pen Tablet and I'm having the same problem. I'm using Gimp & Corel Painter Essentials programs. I get a jagged fuzzy line each time. I've tried a resolution around 300dpi and the problem still persists. I'd like to mention that my tablet feels too sensitive almost to the point where I can't even draw a nice square! Anyone know how to tweak this? I'm running out of options, I might have to just settle for overly meticulous vector art, ugh...
Is this your first tablet? 'Cause all lines on all programs with all tablets tend to be very jagged. They're very... touchy. You can get around this relatively easily by upping the resolution on the pictures that you use, or by editing the correction of the pen on the pen menu.