I think it's great. My signing is very very rusty now, but to me the subtitled words are words that wouldn't be commonly known or spelt out rather than signed (if you know the signing alphabet you can see him signing some words letter by letter) so there is a quick reference to it.
It is good as you can focus on the signing and just flick down to any words which may need clarification so like having someone speaking to you via signing, rather than having to rely on subtitles.
It is good to be able to concentrate on the person as, as Gness said, there is also a lot to be read from facial and other expressions, lip reading etc... and signing isn't as literal as what would be spoken. Expression helped me immensely when I was learning to sign and chatting to deaf people as you can convey so much more than you realise without speaking.