unless is fine for simple conditionals
I disagree, but the reason is rather subtle and my experience shows that most people just won't buy the explanation until they've personally been burned by it. I've seen quite a few people get burned by it by now, and I'm quite convinced.
The "not" that is implicit in "unless" is subtle enough that it can get "lost" when you are looking at code. I've seen several people (including myself) stare at code over and over trying to figure out what is going on because they mentally misplaced the too-subtle "not". The code does exactly the opposite of what they expect and they can look at it and break it down over and over and still not recover the missing "not".
Just a couple of days ago, someone asked one of those head-slap questions in the chatter box to which I responded "because 48 *is* less than 50". They had looked at this very simple code (one simple statement with one simple comparison) over and over and couldn't figure out what was wrong. In this case they hadn't even used "unless", but had started to use "unless" and then changed their mind and used "if" instead. The distinction between the two is so subtle that even after repeated attempts, they failed to notice their mistake.
This shows that not only shouldn't you use "unless" even for simple conditionals, but shouldn't even consider using "unless". (: