The Bull / Jake Mountain system is full of single track and there is plenty of fast downhill throughout. The scenery is great, too. Mountain laurel makes tunnels over parts of the trail and there are creek crossings and waterfalls. The trails are basically split 30% primative road, 30% doubletrack, 40% singletrack. This area can be confusing so be sure to bring a map! Plenty of forest road intersections and bail out points.
Total climb for the entire network is about 3,000 feet, though there are several loops you can make with as little as a few hundred feet of climbing each.
Update 7/11/13 by mtbgreg1:
Some people have mixed feelings about the Bull/Jake Mountain trail system, one thing is certain: this trail system is one of the premiere mountain bike destinations in North Georgia. Sure, there are better trails in North Georgia, but there are definitely worse trails, too. If you're looking for riding in the Dahlonega/Dawsonville/Helen area, this is undoubtedly the place to go.
The trails have changed significantly in the past couple of years, and they will continue to change. The US Forest Service has been rerouting many of the trails that were built unsustainably. Again, these reroutes aren't without their detractors (as the new trails are very buff and smooth), but in general the riding in the Bull Mountain area is now the best it has ever been. And with new trails proposed in the coming years, the riding here stands to get even better.
Right now, the trail system can be split into two different halves. The Jake Mountain side of the system (which includes trails such as Jake Mountain, Black Branch, No Tell, Turner Creek and Moss Creek) is, on average, flatter and smoother than the Bull Mountain side of the trail system. If you're not from the mountains, don't let that fool you, though. Despite being flatter than the Bull Mountain side, nothing out here qualifies as "flat": you can still easily climb a couple thousand feet in less than 20 miles.
On the Bull Mountain side of the trail system, the trails on average are steeper, with longer climbs, and are rougher and more technical. The Bull Mountain, Whoops, and Lance Creek trails comprise this half of the system, and the Jake-to-Bull connector and Jones Creek Ridge trails function as the links between the two halves. Consequently, Jones Creek Ridge and Jake-to-Bull have more climbing than the Jake side, but are smoother than the Bull side.
Without a doubt, the full Bull Mountain loop is the main challenge, with long, steep climbs, fast descents, and some decent rocks to keep things interesting. The Bare Hare trail at the top of the Bull Mountain loop also reaches the highest elevation of any of the mountain bike-legal singletrack trails in the area, providing some pretty cool mountain riding.
When you come up here, be warned: this is one of the most popular equestrian trail systems in North Georgia, so be prepared to yield the trail to horses. But don't let that deter you from experiencing the awesome mountain biking!
For a loop route with the most singletrack riding and the least gravel road, and absolutely NO backtracking, be sure to check out the map for the Dirty Thirty recommended route.