Make sure that a Macintosh serial cable is included - since most modems can be used on Macs and PCs, you could end up with a PC serial cable. A few modems may not be compatible with PCs due to non-standard ports or a lack of drivers, though that's seldom a problem for most Mac users. If you might want to hand your modem down to a relative with a PC, make sure it can be used with the addition of a PC serial cable or appropriate driver software. PC Card modems have other issues related to how they connect to standard RJ-11 telephone plugs. Most PC Card modems include a small dongle that plugs into the outside edge of the PC Card and provides an RJ-11 jack.
Dongles work fine but are easily lost.
Although this strategy has no parts to lose, it's more vulnerable to accidental damage. All modems are not created equal, nor are the companies doing the creating. Some brands of modems have better reputations than others, and sometimes you do get what you pay for. Pay special attention to anecdotes surrounding tech support. Because there are so many variables surrounding modems, you'll want to talk to qualified tech support representatives if you experience problems.