Outside of taxation, a buy back is essentially equivalent to a dividend that you're forced to reinvest. At the end of the day, all of your shares are worth a smidge more of the company. Certainly, the price could go down after a buyback, same as if you reinvested the dividend.

The benefit for shareholders is that the shareholder can decide when to recognize that as a capital gain; the negative for shareholders is that they can't recognize only the portion of their capital gains related to the buy back.