I've been dying for a MIFI for some time now. Finally bit the bullet and bought the previous deal posted onto here (39.99 device, 15/month for 5gb). I RECEIVED THE DEVICE TODAY, and thought I'd give my shot at customer services to reduce this price.
I just spoke to retentions (even though I've been a customer for 20 minutes) who gave me the same 5GB plan at 50% off if I signed a 6 month contract, after which it turns into a one-month rolling contract at 7.50/month!!
Basically, 1/2 off the deal. Plus, you get quidco (mine's tracked already) bringing down the up-front fee.
Can't find anything better than this - 5gb of 3G for 7.50!!
Edit: speedtest.net speed test from central London yields 6.2MBIT.