Not really, at least it wasn't when I tried it a good while back.
Their current plans start at £6/month and go up to £25/month. All include unlimited calls and texts.
A few months ago they doubled the £20 plan from 20GB to 40GB, then doubled it again to 80GB. I tether my main work computer in the evening (for the hour or two I can get programming work done these days), and have music streaming most of my waking life. Only when I actually try do I hit the 80GB within a month.
The trouble with the Always On* plan is the *...
With Always On data, you get 80 GB of data at full 4G speed. After 80 GB of data used you'll experience a reduced data speed of 384kbps from 8am to Midnight. You may notice that activities which require high amounts of data, like HD video streaming, will be slower.
384kbps. Note that the b is lowercase. That's only about 8 times faster than half decent dial up speed. Typically I'd get about 30mbps, and you really notice it when you lose 99% of your internet speed.
The one time I tried the Always On plan I ended up buying the £20 plan within half an hour of running out of the fast internet. It just wasn't worth trying to put up with.