I flew the new A350 earlier in the week, Heathrow to Toronto. The wifi was OK for general browsing, email etc, dropped out a few times this is the new/alternate system on the A350.
Pricing is by amount of data. During the flight my personal phone used 2 x the small (25Mb for £4.99 I think) package with the second expiring about an hour before landing. Use was browsing, WhatsApp and about 15 emails downloaded, no large attachments.
As they (annoyingly) dont do a multiple device option I also connected my work phone, this into the biggest package (150Mb for £17.99 if memory serves), again this ran out about an hour before landing.....probably sent/recieved 60 to 70 emails, some with PDF/Excel attachments.
All in it seems overly pricey and I think the 'whole flight' packages offered are a better offer. Whilst in the US this week I've done two internal US flights with AA and will fly back to UK with them tonight, their packages seem better value and more stable.