Carriers in HOI are either useful or useless, depending on your nation, and your strategic objectives.
The 3 nations that need CV's are Japan, USA, and UK.
As others have noted, the units you have on a CV will depend on what it is being used for, and what enemy air can threaten your fleet.
The key with CV's is range. The give a player the ability to project airpower on seazones that his land based air cannot reach. They are flawed, have limits, and won't generally stand up to land based air, but in certain situations they are incredibly useful. Players that say they aren't have simply never played Japan or USA with a human opponent, or have never had to deal with a german u-boat menance in the central Atlantic, beyond the reach of land based units.
Personally, I combine 4 CV's, preferably models with 3 air capacity, with 8 DD's. This gives you a fast force, with good anti-sub capabilities. Those 4 CV's can carry 12 air, which as long as you are not playing a 'CAG only' rule, should be used with a mix of medium range fighters, and naval bombers. The MR fighters have naval attack stats, and are good enough to fight off any enemy air attacks, or at least allow you to retreat in good order.
All of this is of course, largely superflous against the AI, but I promise you, in MP I have seen players send fleets beyond the range of their aircover, only to have these same fleets with all their fine battleships and cruisers sent to the bottom. Airpower is key in naval combat, and CV's fulfil the useful purpose of extending the range at which you can deploy that air. In the med, or North sea of course, range is not an issue, and hence I would never bother building CVs as Germany, Italy or the USSR.
