Nice to see a thread about this one as I just finished playing it.
First, let me say that obtaining a copy of this game is about impossible these days, and then, only if you are will to shell out a fortune. A month or two ago, a copy on eBay sold for something ridiculous--$150-200 IIRC. You might try GameTZ--at one time, there were a couple of copies listed, but you'd probably need to offer something equally rare.
As for the game itself, I really enjoyed it.
It *is* alot like Myst, both in appearance and gameplay. There's alot of "walking" around on an island filled with beautiful landscapes and curious ruins. I like the backstory alot better, too. Sure, alot of it has to do with Celtic mythology, but it has a nice bit of gritty realism in it as well, and a comparison to some elements of the film "Raiders of the Lost Ark" isn't a stretch. I wasn't expecting that part of it, and it really drew me in to the plot and cleaned things up for me.
The puzzles were pretty clever. Not too hard, but not pushovers, either. I did need a walkthrough for a couple of them.
You don't meet any characters, but rather read about them through diaries and journals. This may not be for everyone, but I very much enjoy this type of game.
Celtica is short, and you end up wishing for a few more locations to explore--particularly buildings of some type. In retrospect, a few more "red-herring" objects to read and/or tpy around with might have helped as well.
But I would say that if you enjoyed Myst, you'd like Celtica, too.