How long does it take to earn back the investment of a miner? I was strongly considering buying one of the 25 GHash/s miners and hooking it up to my laptop or desktop when not using them, as in, leave the miner running overnight or while I'm at work.

The mining profitability calculators confuse me. If I join a pool and mine the way described above, would it be profitable?

How long would it take to earn that c. $800 back?
You can run the miner all the time, not just overnight or while you're at work.  It doesn't use hardly any resources on the laptop... all the heavy lifting is done on the miner.

It's very risky to buy a miner.  The miner manufacturers often take a long time (i.e. several months) to deliver your order.  While you are waiting the mining difficulty is going up and the manufacturers move on to new more powerful models.  As a result, by the time you get your hardware it could be well on its way to becoming obsolete.  Keep in mind that a mining rig is not useful for any other purpose.

It is a very real possibility that you will never make back your investment unless the bitcoin price goes up a lot.  And if you want to bet on the bitcoin price going up, you can simply buy bitcoin and save yourself the whole mining hassle.