My last laptop had a serial port, but only when it was inserted into its bulking docking station. My laptop of the past 2.5 years doesn't have one. I used to spend a lot of time in datacenters wanting to configure devices over a serial console, so as a result I've been using USB to serial converters for some time. And I've never really been that happy with them.
My first device was a Belkin, using the belkin_sa driver under Linux. It was free (always a plus) and was fine for basic 9600 8N1, but didn't work with my eTrex GPS or my smartcard reader. I just assumed they were picky about what they wanted to talk to - I didn't need to use them often with my laptop and my desktop has plenty of serial ports thanks to a 4 port Moxa card.
I then bought a cypress_m8 device, as Simon had one and claimed to be happy with it. While again it was mostly ok for datacentre usage it didn't work with the GPS/smartcard reader. Also I'd sometimes see dropped characters especially when driving it at 115,200.
I just assumed this was all a limitation of the devices; I'd have said it was the fact they were cheap and nasty but I know the retail price of the Belkin wasn't that cheap. I asked around on IRC and people said they'd had no issues with pl2303 based devices. So I bought one from DealExtreme for a bargain price, figuring it was worth a try. And it was. It works with my smartcard reader and my GPS. It doesn't drop characters anywhere near as readily, even at high speeds. And it was cheap as chips. I've seen others complain about the pl2303, but it's definitely the best of the 3 USB to serial chipsets I've tried.