At its best, it works very well, and can do almost everything that home broadband can, e.g. streaming videos from YouTube. However, to manage this you will need to be in a strong 3G signal area for your service provider, and there must also be adequate network resources for the number of people trying to use the service.
I use Vodafone Mobile Broadband, and on a recent trip found that mobile broadband worked well for general surfing and catching up on e-mails. However, whilst you can usually get 3G coverage in large cities, if you are out in the countryside then often 3G will not be available, in which case your mobile broadband service will revert to GPRS, and this is extremely slow, much slower than old dial-up and not much of a surfing experience.
Mobile broadband speeds quoted in advertising are the maximum speeds theoretically possible. So, whilst you may hear speeds of 7.2 Mbps quoted, which is a similar speed to home broadband, the reality is that the actual maximum speed you will manage will be a lot lower, say around 2.5 Mbps, and typical speeds will be even lower than that. If you decide to get mobile broadband then have realistic expectations about the speeds you will obtain.
When in a 3G area it will usually be fast enough for general surfing and checking your e-mail, but more intensive tasks such as video streaming or VOIP may not always be possible. Mobile broadband also has a slower response time than fixed line broadband, which means that the ping (time taken for data to go from your computer, to the server, and back to your computer) is longer.
This is particularly noticeable for online gaming, which means playing demanding action-intensive online games may not be feasible with your mobile broadband, although adventure games such as World of Warcraft would still be viable.