Not for everyone, but if you need a Windows Mobile phone (e.g. for Exchange synching) and the Touch HD2 is too expensive, this is worth a look. As a low-volume user, I went for the Combi 15 option (100 mins and 100 texts) plus Web'n'walk (1Gb), but there are higher-volume offers there too, with higher auto-cashback. Also some cashback-by-redemption offers for those who like to gamble.
£20 per month for 18 months, minus £55 automatic cashback, minus £25 Quidco or Topcashback, equates to a possible term cost of £280. Pretty good for a phone that costs £270 sim-free.
For comparison, if you go to T-Mobile direct, they will charge £99.99 for the phone on this tariff, though offer a small discount on line rental (£18 instead of £20). Term cost (after £35 Quidco) would be £388.