I'm biased, but entrepreneur to entrepreneur, here's my honest personal opinion. I won't pay for bookmarking. It's too easy for me to cobble together my own solution, and besides, there is already a lot of bookmarking apps that are free.
I think there are certain things in life that the market is not willing to pay for. Bookmarking may be one of them. On a related note, I'm looking around at all the all the micropayment solutions out there on the web. Supposedly the idea was that you can read a blog post, and if you like it so much, you can tip a penny if you want. I haven't heard too much success in that regard .. but I suspect that people aren't willing to pay. And as an entrepreneur, it's tough to change the market (impossible?)
To be able to charge, your value proposition has to be not just incrementally, but way waaaay significantly "above the rest" such that it's just such a disadvantage for me _not_ to give you my money to use your product. Use Delicious as your baseline. Delicious is free. So anything you offer that is also offered by Delicious, you can't really charge for it. What do you offer on top of that that is worth people opening up their wallets?
If so, I'd be interested to hear what you have.