If he doesn't regret doing it, I regret having watched it.
I acquired the album with that song when I was 14 or so, about the end of the first run of ST:TOS. I enjoyed it. There were other songs on the album that were far worse. He did a second album. But you also have to think of Shatner's singing career and compare.
I notice that Bill Shatner is starting to take himself less seriously these days. The Comedy Central Roast a while back had some great moments. Including at the start Nimoy telling him not to do it.
I think both of them are at the point where they realize that whatever they do, the legend is intact. Some "stars" think they've reached that point when they have far too little history behind them. The results are often tragic. Or hilarious. Depends on point of view of a given debacle. (For instance, I find most suicides of punk or grunge "rock" "stars" extremely amusing. Their brain-damaged fans think there was a bad thing that happened (but no two out of seven can agree what it was).