Xmarks is great but I don't think it's a service I would pay for.
The core service remains free. Like Lastpass, you will have to pay if you want advanced features like Android and iPhone apps.
The service will remain free, and we're happy to have you as a free user. Our plan is to continue to add mobile device support and ancillary services on top to convince you that maybe $1/month is worth it. We only need to convince a few percent of people to make this a success.