With all of the "Doohan's accent sucked, the guy in Scotland says so!" BS accepted, let's remember that on American TV - particularly in the 60's - accents were were all pretty much ballparked. And, speaking as a stage actor, they still are. Unless you're aiming for a particular dialect within the culture for character purposes, you just tried to get certain inflections. The way the voice raises at the sned of a word or how you roll an R or whatever. Just like "hey Jamie Bamber does a great American accent!" He's not doing New York or Boston or Texas...he's doing a bland "sound like the guys on TV" accent. The Russian accents all over the airwaves in the 60's weren't all true to life. As someone said way up top, we have to be able to UNDERSTAND what the actors are saying. My sister in law dated a guy from Scotland and I couldn't understand a single f'n word he was sayin' more of the time. I think once I made out, "Skut, yer a ghud mate." Because I got up at 4am to watch a Soccer game at a bar with him.
Was Doohan's accent good? Yes it was, for the needs of Star Trek and most people in the US - where it was made for - was fine with it. Was it accurate? Apparently, it depends on who you ask. But, jeez, some of you are really hard on him and Koenig for their work.