They have a great idea and a great service but there are two things I just can't get past.
1) Lack of support for normal file formats vcf/txt/xml whatever due to use of proprietary (though published) api
2) Forced cloud sync
through them with 50mb limit (for free service).
If they did something like xmarks/foxmarks where you could use your own server to sync I might reconsider. But as they stand now, despite a published api, they seem to make their users too reliant on the company.
I can definitely see why people like them though, it does appear a useful service. It's just not for me given the above concerns. Of course you could say the same thing about google making people use their "cloud." But hey, it took me almost 10 years before I could trust Google
(still don't entirely!)