I have a Vodafone 3G modem on Pay as you surf, mainly because I don't use it that often but occasionally I do need internet access when I'm away from home.
I have found when I get a 3G or HSDPA connection it runs really well and is quick enough for my needs (browsing, e-mail, remote access). Due to the low allowance (£15 for 1GB) I tend to avoid streaming videos and audio.
However, there are many places I can not get any reception, or at best GPRS. Now GPRS isn't that bad if I'm just using text based e-mail, but I wish that the reception was better.
So I'd say before increasing the speeds, sort out the lousy reception.