That Promise ad has to feature Shatner's worst ever wig.
As for the man himself, generally he's better in early Trek when he's still well respected theatre actor William Shatner rather than The Shat. That's not to say there's not enjoyable stuff later on, it's just the ratio between actually acting and hamming it up shifts more and more towards the later as he goes on.
I don't know if he's ever admitted it, but I really hope around the time he did Rocket Man he was on lots and lots of hardcore drugs. That'd explain it.