I haven't seen the full details yet, but in theory T-Mo's idea of £4 for a day of 3G (and I assume HSDPA) access from a laptop with a USB modem sounds pretty impressive.
That's finally crossed the threshold of usability / value versus a typical London Internet cafe's £2 per hour, and could therefore appeal to ad-hoc users who've got access to a laptop but are loathe to sign up for a monthly contract. It also makes a lot of the current pay-per-use WiFi services look very expensive - for example T-Mobile's own £5 / hour at UK branches of Starbucks.
The interesting question is whether this will be available to travelling business users as well, enabling them to just buy a suitable SIM card when they land at Heathrow, enabling a massive saving against 3G roaming fees.
As a side note, I'm definitely seeing HSDPA becoming a viable alternative to both WiFi in the UK at the moment - although this largely reflects stupidity among WiFi hotspot operators on reaching sensible pricing levels. They've squandered a 4-year lead on the cellular industry.
HSDPA is also making inroads into home broadband in some niches in the UK - I've had a couple of friends recently mention they're looking at it rather than ADSL or cable. It's especially attractive to young urban professionals, immigrant workers or students who have a high chance of moving home more than once a year, and who therefore don't want a 12-month broadband contract. In some cases they'd also have to pay £££ to have a BT line reactivated & reconnected at a new place, which definitely makes the process more drawn-out and expensive.