Well, I'm on day 3 now after dumping 3 Mobile Broadband for not being able to use their service or mediocre speeds.
At first I was a bit surprised to find VodaFone downscales images, which can make some websites look like rubbish. However, as you'll notice from hovering your mouse pointer over the image, Shift+R will reload a specific image and Ctrl+F5 will reload the entire page in full resolution. So it is worth the bandwidth savings over time!
By default, VodaFone also has content filtering enabled. So if you're into that kinky stuff, you'll have to disable this. But it also filters a few fairly innocent things, like Flickr, which was something that wanted me to remove the filtering.
Their "My Account" website isn't as good a 3 Mobile Broadband, I must admit. It seriously needs some tweaking. For instance, I could not enable the Monthly Billing service for my mobile broadband service (kept saying there's an problem on their side). Also, trying to remove the content filtering as described above, I found that the page asking for your credit card details is *insecure* (Not HTTPS / SSL secure). After biting my tongue and risking my details, found out they specifically need a credit card, not a debit card. I'm stingy and don't use credit cards

So I'll have to call them about this, I guess.
For the rest, I've been able to browse the Internet hours on end without issues or slowdowns, which is the whole reason why I left 3 Mobile Broadband. And should I go over the 3 GB limit (which is an actual possibility now) it doesn't cost me £1 per MByte, but £15 per GByte - still not cheap, but certainly beats £100 per GByte!
I just did a little speed test. Due to caching, results aren't always accurate (for instance, one site gave me download speeds of 223 MByte (!) per second. Ah, I wish. But I've found a fairly accurate one, which with this link will include the results I achieved:
thinkbroadband :: Broadband Speed Test Results
2.2 Mbyte per second download speed... Finally a service that actually comes near to its advertised speeds! (angry look at 3 Mobile)
Apart from some of the issues with their website, I'm very satisfied with VodaFone and sorry I didn't drop 3 Mobile Broadband sooner. I hope this will remain the same towards the end of my trial period!