Look for models that have stainless steel and all-metal parts, and that also have at least five different thickness settings. If you can, take the machine out of the box and try inserting the crank arm. It should fit snugly and the arm should be easy to turn. You should feel some resistance while turning, but it shouldn't take huge muscles or feel like the rollers are free-wheeling inside. Also, make sure you have a counter with a few inches of lip or a stable table where you will be able to set up the pasta maker.
There are dozens of hand-crank models on the market ranging in price from $25 to $150, which makes picking the "best" one feel like an impossible task. Atlas and Imperia models tend to get good reviews, though we haven't had the opportunity to try them ourselves. We personally own an older model Ampia Pasta Maker, which has worked great so far! Really, as long as you buy your model from a reputed dealer like Williams Sonoma or Sur la Table, you can feel pretty safe that you're buying a decent product.
In culinary school, we also had the pleasure of using the KitchenAid pasta attachment. If you already own a KitchenAid mixer and plan on making a lot of pasta in your future, we think this attachment is well worth the investment. The attachment fits right into the head of the mixer and the mixer's motor turns the pasta rollers, allowing you to use both hands when working with the pasta. As judged by the antics of novice chefs, it's very easy to use and can withstand extended use without jamming or breaking.
We don't know much about other mechanized counter top pasta rollers. Truthfully, a lot of the ones we've seen don't really impress us. They seem more geared toward selling an easy-to-use, touch-button product than making quality pasta.
What kind of pasta maker do you have...or wish you had?