Oh Lord, have some fun!!!!
Yes, buy one if you don't have NFA toys. Personally, I like the one made by Fostech - would buy one if I could find one used in the $300 range - this is an all metal/aluminum version for the AR - there is also a good one made for the AK by Bumpinski or something like that.
If you don't want to use it to bump-fire, then it can be locked down for semi-auto fire. Spend the freakin money now. If you get bored with it, sell it and make a profit in the future. If you think someone will be making a decent one in several years that you can buy, you are probably wrong. If it works, ATF will put them out of business....and they work.
People spend $200 just on a stamp to be able to spend $10,000 for a gun that does basically the same thing. $300 - $500 is dirt cheap thrills.
Yes, be careful not to over-pay....i.e. be patient, but if you even think you like full-auto then just do it.
Let's get real here - I have only two serious guns. One is a Ruger LC9 - pocket 9mm for personal protection and the other is a Savage 110 30-06 for hunting. Nothing fancy, fast, shiny, or sexy about either. The rest of my guns are basically toys for my enjoyment. Anyone who thinks its stupid to add a new widget to a gun to make it go BOOM BOOM BOOM BOOM faster is has an overstated opinion of the importance of their firearms stash. Who knows, the Bumpinski (or whatever) might even be a useful tool in a SHTF scenario.....but then again, who cares - boomboomboomboomboomboom is fun!
What do y'all think about this for a tag line: "Board Certified Gun-Clown".