According to Boston University finance professor Allen Michel, when a company announces it's buying back stock, that stock tends to outperform the market by 2%-4% more than it otherwise would have over the ensuing six months.

But over the long term, multiple studies show that buybacks actually destroy shareholder value. CNBC pundit Jim Cramer cites the example of big banks that bought back shares in 2007-2008 -- just before their stocks fell off a cliff. Far from buy signals, Cramer calls buybacks "a false sign of health ... and often a waste of shareholders' money." Indeed, the Financial Times recently warned: "the implied returns over a period from buy-backs by big companies would have been laughed out of the boardroom if they had been proposed for investment in ... conventional projects."

So why run buybacks at all? According to FT, management can use them to goose per-share earnings, which helps CEOs earn bonuses based on "performance." Also, the investment banks that run buybacks earn income and fees from promoting them. But you and me? Unless the purchase price is less than the shares' intrinsic value, we miss out.

And we're about to miss out again.

Two bad buybacks
StreetInsider.com keeps a running tally of which companies are buying back stock, and how much they're spending. SI is too polite to accuse companies of wasting shareholders' money, of course -- but I'm not. With SI's help, I've uncovered two examples of popular stocks that I believe are squandering shareholder dollars on ill-timed buybacks...and one stock that isn't.

American Capital (Nasdaq: ACAS)
Three months ago, I criticized American Capital's decision to repurchase 8.4 million shares of stock, even as I admitted that at just 3.8 times annual earnings, the shares looked anything but expensive. With revenues on the decline, AmCap had been forced to cancel its dividend, and as I argued at the time: "this company ... can't afford to pay dividends right now. [So why was it] spending nearly $60 million buying back its dead-in-the-water shares?"

Ordinarily, you'd expect buying activity to support the stock, as management stepped in to buy anytime share prices dropped. So how has that worked out? Well, over the past three months, AmCap shares are up 5%. That sounds good, but it only paces the performance of the S&P 500, also up 5%. Worse, we recently learned that AmCap bought another 5.5 million shares in Q1 2012. Unfortunately, the prices it paid averaged $8.79 per share -- more than 4% above today's going rate.

If at first you don't succeed, fail, fail again.

Goldman Sachs (NYSE: GS)
And speaking of failures, let's turn to Goldman Sachs. The most (in)famous banker of the 2000s is getting pilloried today in the pages of The Wall Street Journal for letting Berkshire Hathaway (NYSE: BRK-A) (NYSE: BRK-B) outfox it on a bond trade involving debt of struggling newspaper company Lee Enterprises. But that's not the only reason to doubt the stock's prospects.

In a cryptic note, Goldman confirmed last month that after the most recent round of bank stress tests, it now has a green light from the Federal Reserve to proceed with a planned "repurchase of outstanding common stock" of unspecified size. Depending on the size, this could be a disaster of either minor or major proportions for its shareholders. Consider: At $119, Goldman shares currently fetch the princely valuation of more than 26 times earnings, while rival bankers Morgan Stanley and JPMorgan cost less than 15 and 10 times earnings, respectively. Yet according to analysts who follow the stock, the fastest Goldman can hope to grow its earnings long term is about 15% per year. The resulting PEG ratio of nearly 1.8 suggests Goldman is overpaying for its shares.

Don't make the same mistake yourself.

Apple (Nasdaq: AAPL)
Now, I don't like to end this column on a down note, and fortunately, I have spotted one company out there that may do better by its shareholders: Apple. Perhaps you've heard of it?

Last month, it was Apple's initiation of a $2.65 per share dividend that got all the headlines. Less noticed, though, was the i-everything company's announcement that it will also buy back $10 billion worth of stock, beginning on Sept. 30. At first glance, I have to say I like the idea. Priced at less than 18 times earnings (and an even more attractive ratio to free cash flow) and growing at 20% per year, Apple shares look priced to move. My one concern here (I have others elsewhere) is Apple's acknowledgement that a prime objective of its buyback plan is to "neutralize" the effects of future stock dilution from employee equity grants. To the extent Apple uses its buyback authorization to buy undervalued shares and benefit outside shareholders, I like the deal.

But to the extent that Apple spends $10 billion to mask the effects of stock dilution, I don't.

Looking for more great stocks priced at "buyable" prices -- without the stock dilution caveats? You're in luck, because we've just found five of them. Read all about it in the Fool's new -- and free! -- report: " 5 Stocks The Motley Fool Owns -- And You Should Too ."