I've been using giffgaff since December. The signal coverage isn't great (but this seems to be a general issue in UK) and the internet connection sometimes isn'

I've been using giffgaff since December. The signal coverage isn't great (but this seems to be a general issue in UK) and the internet connection sometimes isn't perfectly stable. Personally, I find this tolerable and the prices are great. The internet connection is genuinely unlimited - but you'll get a warning if you constantly use more than 100MB/day.