I planned to do an LTTP for PlayStation Now a couple of months ago but never did. I did do an
LTTP of a game I played through entirely with it, so here's my experience:
I found it works really well for either slower-paced titles, games with distinctive (non realistic) art styles or 60fps games, or a combination of the above.
I tried a number of games but eventually used my subscription to play through the entirety of Atelier Ayesha, which is a game with an unlocked framerate and 60fps menus. I found the experience very good - 99% of the time I forgot I wasn't playing it natively on a PS3. The ability to log into PS Now using a desktop or laptop is also incredibly useful.
I tried other games: Sonic Generations was a disaster due to the 30fps and extra input lag. Daytona USA looked good at 60fps, but you could feel the extra latency. Still playable though.
Video quality was mostly fine. Grittier looking games like Motorstorm Apocalypse fared less well, but Daytona, Atelier, Gravity Rush Remastered all looked good enough, and the PS3 games were close enough to their original 720p native equivalents.
I'd recommend it if there are games on there that you want to play through and if PS Now is way more convenient than setting up your old PS3. The issues I had with it were with the poor content discovery (the actual app is fine, a mirrored experience on PS4 and PC) and the region-specific content offerings. For example, Tales of RPGs are available in America but not Europe for some reason. The official website for PS Now doesn't even list all available games either.