The refinancing boom means a sudden surge in new business for a lot of mortgage brokers. The typical refi costs a homeowner maybe $2,000 or so in costs, including fees.

Brokers may be among the few making out in this economy -- which is ironic, because some (repeat: some) are the villains who got us into this mess in the first place.

But before you join the stampede, it's worth asking: When does it make sense to refi?

If you are planning to move or even pay off your loan within the next few years, refinancing probably makes little sense because you won't be paying monthly bills long enough for the savings to cover the costs.

On the other hand, in some circumstances, refinancing is pretty much a slam dunk.

If you plan to stay in your home for years, and you are currently in an adjustable-rate mortgage, you should strongly consider a refi. ARMs are incredibly dangerous -- the financial equivalent of Russian roulette, but with multiple bullets. Refinancing into a 30-year fixed-rate loan may not cut your current monthly payments by much, but it gets rid of the risk that those payments will suddenly skyrocket.

Refinancing also usually makes sense if you are currently paying a much higher rate, though few homeowners are any more.

As a rule of thumb, Greg McBride, economist at Bankrate.com, looks for a payback period of a couple of years. "Generally, if you can earn the costs back within two to three years, and it's a home you're prepared to stay in for much longer than that, it's usually a good thing," he says.

But if the savings are more marginal, you need to do the math.

Some mortgage brokers will tell you how much interest you will save "over the life of the loan" if you refinance.

It's usually a very large number. But it should also be taken with a grain of salt.

First, that number ignores taxes. Mortgage interest is deductible from your income tax. So paying less interest may mean you will pay slightly higher taxes.