My first table was a 3x6 bar table. Heck, for a while I had it shoved into a corner of my living room and I could only shoot from two sides. I think any pool table is better than no pool table. In the case of a 3x6, I think you'll find it makes for pretty good practice of cue-ball control and cluster breaking. If you play a lot of 8-ball on a 3.5x7 then practicing on a 3x6 could help your game a lot. If you play a lot of 9-ball then it won't help you as much since cue-ball control is not as critical, but again, even a small table is better than no table, IMO.

Another thing about smaller tables - there aren't any long shots (duh) - so you'll probably have more runouts than you might have on a bigger table. This can really help your confidence and make solo practicing more fun.

I wouldn't pay much more than $300 or so for a 3x6 table. Most of the time when these things go up for sale it's because either the owner (a)bought a bigger table, or (b)just didn't want the table anymore. In either case they're usually pretty anxious to sell.