How often will you use it? If you plan to use your electric knife on regularly basis, then it's worth spending more for one that's well made and has a long warranty. On the other hand, if your electric knife will only make an appearance once a year to carve your Thanksgiving turkey, then a cheaper model should meet your needs.
What does it need to cut? Some electric knives come with two blades, one for meat and one for bread. If you plan to use your electric knife to cut bread on regularly, then you'll want a dedicated bread blade. If your knife doesn't come with one, it may be possible to buy it separately.
How strong are you? Choose a knife that won't tire out your arm during the time it takes to carve a roast, and one with a switch position that doesn't require you to uncomfortably stretch your hand too far.
As with any other knife, it's important to handle an electric knife before you buy it. Many of the complaints we saw about specific knives concern the shape of the handle or the position of the switch, factors that are best assessed in person. You also want to make sure the knife isn't too heavy to prevent fatigue. Lastly, make sure the retailer you buy from has a flexible return policy.