I think this is an excellent site that really caters to its users. One caveat for child safety is that I've seen some pretty raunchy avatars on some profiles; I'm not sure if last.fm regulates obscene images.
I was a paying site and love it for ages. Then my scrobbling became corrupted and I kept hearing my kids music over and over again in my mixes. This happened around the same time as I moved my entire library to Google Play Music - and now I listen to that exclusively.