I've tried the tables and could never get quite comfortable. I could never get the foot/ankle supports quite tight enough, so it always feels as if my ankle is slowly being pulled apart. What I use are the gravity boots (also called inversion boots). They cinch around your lower leg right above the ankles, and have a hook on them so all you need is a pull up bar - I even used them on my kids swing set in the back yard (not the top tube, the set had a 'trapeze bar' thingy).
From personal experience, the boots have done a lot to give me relief from my pinched sciatic. I would get periodic flare ups where one leg or the other would have shooting pains, especially when sitting down. After using the boots consistently for a while, the flare ups rarely happen anymore.
Like Jim said, try one out if you can. The tables are also easier to use for some people. With the boots, I have to do an inverted sit up to get back up where I can unhook when I am done.