well I never actually owned leo's castle, but from what I've seen I don't like the set very much. As many have said the design is just terrible; too many colors in too many places, not to mention that it hardly functions as a castle.
I don't even like the parts that much, the stained glass is rather worthless imo- there are techniques that give much better look. I'm not a fan of the half tower pieces as well, hard to integrate with a moc.
The only really nice rare pieces are the 75 convex slopes.
I suppose the minifigs are cool, minifigs aren't a huge concern for me though.
If I had to give a number value- maybe 2or3 out of 10. The set did so many things wrong, its hard to find whats good about it.
Even for a kid I think it would be disappointing to build, from the looks of it the construction would take all of 10 minutes. - I remember building the black seas barracuda as a kid, it was tough and took awhile but it was really rewarding to see the finished ship.
6090 was a very nice castle- probably the last good medieval castle lego made. I think it was also the biggest? It does mark the beginning of the BURP era though