We picked seven of the most popular computers that retail for under £250 (one is just over £250), and took them for a couple of days' diving off Land's End to find out. All of them performed remarkably well. We checked their settings before taking them into open water, but all of them could have been used straight out of the box. It was also good to report that, without exception, they had all been set to metric rather than imperial measurements. The main differences between the models are their algorithms. Some allow longer dives or are less conservative, depending on how you view things. Some cram as much information onto the display as possible, while others require you to push a button and activate a second display. All models have user-replaceable batteries and the majority come with screen protectors.