Similar for me.
I've been an Ovivo customer for about a month and a half now. Was rather annoyed by the change away from the packs, as they were really helpful to buy data when we were away from home for any length of time.
Never saw a single advert when I was with them the whole time. I did play with Onavo (data compression app, uses VPN for this, so effectively, no ads would get through) for a while, and checked via Twitter that this would be okay. Ovivo said they had no problem with it, T&Cs also don't mention VPNs, so I suspect they have dropped the idea of adverts altogether.
Thankfully I was smart enough to e-mail them about my coverage problems at home from the start, and got a really healthy amount of time to port out *and* get a refund of unused credit. Which I've now done.
So now I'm moving to Three, as I want unlimited data, and giffgaff/O2's signal is a little shaky at home... So that's my phone sorted with a 30-day contract.
For Ovivo's flaws though... If I had decent signal at home, I'd have been tempted to stay, and even forfeit the unlimited data, as the customer service was excellent, and the price was right, for the most part.
When the new Nexus 7 comes out, I will be buying the 3G version, and shoving a sim card from Ovivo in it... Just 'cos I can!
Ovivo has it's place, but not for my main phone anymore, sadly.
Also got a Samba SIM, which is pretty good and works well with my MiFi device.
Nexus7 Mk2 with a Ovivo SIM, and Samba with a Mifi will be a killer combination for my time away from home and should guarantee good signal everywhere with a low price of £0/month.