Well I have had the same wondering but it depends, if you're just going to do it as a hobby, how long are you going to mine per day?  From what I've heard the investment will come back in about half a year from mining 24 hours per day in a pool IF nothing dramatically changes, which probably won't happen.. So if you want to invest as a hobby, I wouldn't go for it.

It really depends on what your goals are.  For the price, you will have a hard time ever breaking even on a single erupter - which is fine if you know that going into it.  You might be able to break even through mining something for a small period of time and then selling it (either asic or gpu).

If you want to dip your toe in mining and have any reason to want a higher end graphics card (gaming, cad, whatever), AND you already have a pci-x slot open and psu that can handle it, buying a 7950 new or used is a nice option.  You'll spend more, mine just a bit more than an erupter, but hold your value more.  Also, you can explore ltc mining later if you want.  If you want to learn the mining tools and be a part of the network that's a great place to start.  My 0.02USD.