Yep, they get pretty big (1.8m, 60 kg), and since these are farmed fish, one would assume they've already been selected for large size and fast growth. The ones in Waitrose were small, about 40-50 cm long. Quite handsome fish, and I'm sure they look wonderful in aquaria.
Didn't buy one I'm afraid... will try one out some time. I'd imagine they taste much like other predatory perciforms such as seabass or pike-perch, i.e., yummy.
(The downside is environmental impact. Farming predatory fish needs 10 kg of fodder fish (to make fish meal pellets) for every 1 kg of predatory fish sold at market. The fodder fish is usually marine stuff and includes things like sand eels and small clupeids. There's some evidence that overfishing these things has a serious negative impact on the wild animals that feed on them, such as seabirds; certainly, overfishing has caused a collapse in stocks of sand eels at certain places around the UK. Omnivore fish, like tilapia, don't need fish meal, and will eat pretty much anything.)
The rumour is those Takifugu puffers being traded periodically are siphoned off fish farms, where they're being researched for things like up-market sushi. You only need to see how regularly things like tilapia, Pangasius cats, and channel cats turn up in aquarium stores to see that species common in aquaculture have a habit of being offered as pets.
Cheers,
Neale