I know this is an old post I'm responding to, but the Belt Line is a creature in regular evolution, so here's an update.

If you imagine the beltline like a watch (and it's more like that Dali watch than anything else), there are portions that are complete, and many discontinuities.

The Northside beltline runs from about 11:00 on the imaginary watch directly north ... it doesn't attach to anything and isn't particularly long. I can't report from personal experience. Haven't ridden it yet. It looks like there are some street riding portions, which in Atlanta is nerve-wracking.
The Westside beltline runs from about 8:00 in a southward-ish sweep counterclockwise to about 4 oclock. The portion from 8:00 to about 6:00 is very nice pavement: well built, well maintained, safe and pretty empty. (Some very nice art and some completely befuddling pieces.) ***However,*** the portion from about 6:00 to about 4:00 is gravel, about 2 miles each way of gravel. I don't mind gravel too much, but this is not coiffed gravel--it's DUMPED gravel. *** AND*** right in the middle of the gravel portion to add insult to injury, there's a break in the trail to cross a street. To get to the street crossing, you have to carry your bike up and down about 30' of very steep landing-less stairs. It's a true PITA and the only way around it is to hit the residential streets. Luckily the street routes have bike lanes, but see comment above about Atlanta street riding.
The East Side beltline is the most well known and popular. It runs vaguely from 4:00 on the clock counterclockwise to about 2:00, some of it through Piedmont Park. It's well groomed, has some nice art, has lots of human scenery, plenty of shops and restaurants, and is unbelievably crowded at all times of the day, every day of the week. It's practically unrideable on a holiday or weekend. The good thing about it is that it connects to the Freedom Parkway trail, which runs more or less east-west, bisecting the East Side Beltline. Another great trail, about 5 or 6 miles, nowhere nearly as crowded. There is a street connection to the Southside beltway, but I haven't yet found it.

My biggest comment about the beltlines is that there is no comprehensive map of status and routes. The Beltline folks don't seem to think it's important--or have hidden it out of sight on their website, which seems mostly a money-raising site--and nobody has stepped up to do it, as they have with Silver Comet. Recently there was a report that it will be finished by 2030, which if I do the math right, is another 9 years. I guess I can take my motorized wheelchair to do a circumnavigation in 9 years.