There is a big difference between 3g and HSDPA (3.5g). 3G limits you at best to around 360kbps whereas HSDPA is around 3.5mbps.
If your in a rural area then you may be limited to a 3g connecton so you wont get a decent throughput. If you are in a 3.5g area (most cell providers will have a post code check) you will be able to achieve proper broadband speeds.
Another issue is contention, because it is a shared radio frequency not a dedicated set of wires the more people using it the slower it will be for everyone. So I would imagine around densley populated areas the chances of more people using it are higher and they all have to share the bandwidth pie.
I travel alot with my laptop and broadband across the UK and most places (I cant recall one that was not specifically) are 3.5g