Using express directly is very common. Express is basically the Sinatra of Node.js. Because it's somewhat lower-level, it doesn't have a lot of limitations, as the module is really small. It only gives you a routing system, view system, and the connect middleware.

I haven't used Sails, but I've used Meteor. It's pretty cool, not very useful atm because of some limitations. You're stuck with MongoDB (and meteor's ORM-like tool), single instance application, etc... I find it going out on a path than the rest of Node. It's unstandard. It doesn't use NPM for it's packages, so you have to install it differently than every other module.