I frequently use margin at IB in a manner similar to one of the posters above (MrSpendy), which is to say that my living expenses come out of my margin account, and nearly all of my income goes into my margin account, either to pay off loans or to purchase more securities.
Although I do a fair bit of buy-and-hold investing with margin, I think it's most useful as a tool for liquidity. Most people think of margin as a way to buy securities or other instruments with money borrowed from the brokerage using those instruments as collateral, and that is indeed how many people use it. But, you can also use margin the other way around: by first having a sufficiently large amount of securities in your account, and then withdrawing cash (so long as that withdrawal does not jeopardize the amount of equity in your account). I frequently use this feature, for instance, to shuffle around large amounts of cash among my accounts whenever it makes sense.
But, before using margin, it's really important to understand the type of margin (portfolio margin, Reg T, or any applicable house margin rules). It's also important to note the taxable implications of having margined instead of fully paid for shares -- e.g. for starters, you may receive payments in lieu of dividends on margined shares, which are generally taxed at OID rather than cap gains rates.