I have just ordered a new laptop: when deciding which one to get, I dismissed the ones with no serial port!

Back to the question; yes, I've used serial-USB adaptors:
I have an edgeport 4-port box, a D-Link multifucnion hub that includes a serial port, a Belkin multifunction hub that includes a serial port, and an ATEN USB-Serial adaptor lead.
(I don't use them all at once - some tend to stay in my office, some are for site use)

Yes, the issue of true DOS software is a problem.
The Belkin claims to have a driver which should allow DOS apps to use its serial port, but I've never tried it.

Even some Windows software (which ought to know better) stupidly will only use COM1-4 (Hypoterminal is, of course, one of them).

One thing to beware of is that some adaptors (and/or their drivers) will not work if you connect more than one to the same PC. This was true of the ATEN adaptors. :-(
Therefore, if you need more than one USB-Serial adaptor, be sure that it'll work before parting with any cash!

One nice thing with the Edgeport is that it allows you to manually assign the COM port numbers, if you want to.
It is also faster than using a real COM port at the same baud rate!
(If you're in the UK, I might be able to get you one at an advantageous price...)