It is always something of a risk to bring in a heavy vehicle like the Hammerhead. Usually though, I find that the benefits it supplies far outweigh the downsides. It doesn't perform quite as well against heavy tanks as the Broadside, lacking twin-linked, but it makes up for it in what else it can do. Personally, I always take a railgun on mine, and having the ability to drop a large blast anywhere on the field is extremely useful. Since its also a skimmer, it's easy to move up along the board edge or slip around cover to get shots in at more vulnerable armor points. You can often force your opponent to choose between being in cover from Broadsides or cover from the Hammerhead. If you bring a disruption pod along (which you ALWAYS should when fielding Tau vehicles), you can just move a little bit each turn to gain a 3+ cover save due to the Jink rule.
I usually bring in some Pathfinders, so I have to take a Devilfish for them. The beacon it comes with is very helpful for getting your Suit teams in right where you want them. The Devilfish also benefits from extreme maneuverability, and is only slightly less resilient than the Hammerhead. I typically will have it harassing enemy infantry. It's weapons can chew through them pretty easily, and it can even tank shock if you need to kick them off an objective. If you happen to have a Fire Warrior team aboard, you can quickly claim the objective and hold it. Another good use for them that was brought to light recently is the Sea Turtle (viewtopic.php?f=23&t=18706
In the end, it comes down to personal preference, but I've always found that the vehicles are worth it.