USB mobile adapter with my laptop for some months now, and I'm reasonably happy with the service. I say 'reasonably' because it would always be nice to have more speed, but provided there's a decent mobile signal I get a very good connection.
It worked perfectly on a recent trip to Spain - the adapter acted as a normal roaming SIM does, and searched for the strongest signal. I got a Spanish network message telling me that I would be charged an additional fee because I was roaming, and I needed to accept that. As soon as I clicked OK I was connected within seconds, and the downstream speed was excellent.
In the UK I've had no real problems, and for me it's well worth the £15 a month charge.