Most scientific and engineering research follows the same pattern: you spend a lot of time setting up an elaborate thingamajig to test a hypothesis, then you run the experiment, and spend months analyzing the data. Then you discover that you've failed. Lather, Rinse, Repeat. Data-mining research just uses computer software as the thingamajig, so maybe you do a few more failure cycles per unit time than the guys down the hall (who are building a nuclear accelerator to find the anti-bozon -- but hey, at least they get to play with lasers!)
As far as practical advice for someone contemplating a PhD in bioinformatics, mine would be: don't get a PhD in bioinformatics. There are no jobs, and a PhD is generally just a fantastic waste of time (unless you want to be a professor, and are comfortable with the numerous, well-documented drawbacks of that lifestyle). If you feel that you must explore the field, go far enough to get an MS, then quit.
I realize that's harsh, so I'll add some slightly softer, less-valuable advice: if you must get a PhD in bioinformatics, at least have the good sense to do it in a computer science department, so that you can go back into software when you find there are no jobs in bioinformatics. It's a dangerous mistake to get a PhD in bioinformatics from a biology department (ahem...do as I say, not as I do.)
Failing all of that advice, if you really, really want to be in bioinformatics, despite all good advice to the contrary, then make sure that you spend equal amounts of time at the bench and at the computer. If you want to be respected in the field, you have to have experimental results to back up your computer models, and the best way to do that is to run the experiments yourself. Run away from any program or professor who won't let you work at the bench.