Solo and Co-op are very similar. I only ever play Clacks in co-op mode, because the competitive mode seems very very random and things would change too much between turns for you to make any sort of plans.
The co-op variant is a race to send your words before Moist von Lipwig reaches Genua. We've found that the "easy" track (where Moist has a slightly longer path than the "hard" track") is usually a very tight win for the group - maybe we reach Genua with Moist only having 3 more spaces to go. The "hard" path might be just that little tougher to beat.
But it's really a puzzle game - so if you are happy for a solo puzzle game, it works OK. Not sure if you'll want to play it again and again, but it may be good for the occasional puzzle solving.