They don't always work everywhere, or sometimes they're very slow. They also require a host machine and appropriate drivers and can't be easily shared across multiple machines like say, a MiFi or similar little router.
(There are little battery powered routers you can buy to stick a USB stick aircard into, however.)
Also there are old PCMCIA ones, newer CardBus ones, that smaller standard that Apple used, and then USB... USB is the most popular and common, since it's "universal", but if you have a really old USB 1.0 machine you could be rate-limited by USB 1.0 if the AirCard is capable of higher speeds and in a coverage area where that'll work.
I think personally if I weren't tethering to the phone that already has a data plan, I'd get a MiFi from Verizon on 4G/LTE... I've seen two in action and if you're in one of the LTE coverage areas, and the things are smokin' fast. I assume they fall back to lower speeds outside those areas, and still work. Haven't seen that in action yet.
The other night at a meeting, the owner of one of those had his fired up, his phone hooked to it via WiFi and had the info before my 3G iPhone on AT&T could even pull up the first page of the website on the phone's browser. And you can share your MiFi with up to X others (usually 5 depending on the device and carrier) by just giving them your password.
Also the BIGGEST drawback of any aircard from almost any carrier these days (Sprint being one of the only exceptions here in the U.S. I think) is that there's a cap on total data downloaded per month. None except Sprint are "unlimited" and I think I read that Sprint was having to add limits to theirs too. Typically the carriers are offering "tiered caps" nowadays.
If you try to use one as your only Internet connection and watch videos, stream audio, and maybe download an occasional Linux ISO CD or two, you'll typically bust the cap.