One thing to consider when buying a dive computer is the type of battery that it uses, there are two main types; Replaceable and Rechargeable.
Replaceable batteries are the most common and have been used in dive computers since their introduction to the marketplace. They tend to be fairly standard lithium ion batteries found in many types of watches and small electronic devices, depending on the usage they usually last 1-2 years before they need to be changed.
To get them changed we would recommend taking them in to an official service center so they can pressure test them after the battery change.
Rechargeable batteries have only started to appear in dive computers in the last couple of years as battery technology advances these will likely become more common as they offer several advantages over the replaceable one.
The main advantage is that they can be recharged (Obviously..) which means that you'll no longer need to take them in to service centers to have the battery changed which represents a more economical running cost, although the initial cost tends to be a little higher.
The one thing to be aware of is keeping them charged on extended dive trips in remote locations, however the battery life tends to be quite good and can usually be recharged by USB.