Stationary Spin Bike

What specs should I focus on when looking to buy a spin bike? If you have any model recommendations I'd be happy to hear them.

Assuming you already have an aluminum, steel or titanium bike, I would go with http://www.kurtkinetic.com/roadmachine.php or http://www.cycleops.com/products/fluidsquared.htm

If your current bike is carbon fiber, then I would still get one of the above plus the least expensve aluminum or steel bike I could find that matched the geometry of your regular bike as closely as possible.