I've had two WTTW scans, both not due to any concerns but wanting to see if there was a heartbeat before we shared news with family members (in both cases we were due to see my MIL who lives far away at around the 8 - 10 week mark and we wanted to let her know in person but didn't want to do so without some sort of idea that things were progressing as they should).
The first was really great (at their Croydon/Hackbridge one), very detailed, they took lots of time to point things out. They spotted a subchorionic hematoma and explained what that could mean, didn't panic me, gave me plenty of information about what to do if I had any bleeding and gave me some pointers on what to ask at my 12 week scan. As it was I never had any external bleeding and the hematoma had been reabsorbed by the 12 week mark but it was reassuring to be aware of it, and would have really helped to know if I had started bleeding. The scan images were very good quality too, considering the gestation. If it had been bad news I think they would have handled it well - it felt like they had a lot of time for us.
Second time around was this pregnancy (currently 15 weeks) and it was rather different - very quick, we were in there only a few minutes, just a 'here's your baby, that's the heartbeat, all looks fine' and done. This was at their Chessington branch. I didn't mind this much, because I felt more confident as it was my second pregnancy, knew what questions to ask and so on, but it was quite rushed. The picture quality was awful - I know that it can be a challenge when baby is that small, and I wasn't exactly there for the photos but to know all was progressing ok - but you could see there was a marked quality difference in terms of the sharpness of the whole image, not just of the baby, so I wondered if their equipment was less advanced/older etc.
I agree with a PP that they're not comparable with fetal grade sonographers and fundamentally it's a moneymaking thing not a healthcare one (so many extras available to buy in the reception area!!) but I don't think they're necessarily comparable. For me using them was just a 'no concerns, checking in that everything is ok' process, and if I hadn't scanned with them I probably wouldn't have scanned at all, so I wasn't relying on them to spot any particular issues. If I had a history of loss, or a genuine concern about my pregnancy, I wouldn't use them. I think most of their business is to do the 'frills' that the NHS rightly won't do - additional scans for gender reveals and so on.