The V-22 is a hybrid between fixed and rotary wing vehicles, a concept that has been tested at least as far back as the XV-3 of the 1950s. Of course there are compromises in the design and the potential for failures that you wouldn't have in an ordinary airplane or helicopter. However, there are also potential capabilities that you can't get from conventional airplanes or helicopters.

One thing is certain - the military needs some new heavy vertical lift aircraft. The Army has gotten rid of their CH-54 Skycranes and depends on the 1960s CH-47 Chinook design. It's a good helicopter (and reportedly the fastest in our inventory) but has a high IR signature, making it vulnerable to shoulder launched missiles. They keep upgrading them but I don't know if any new ones are being built. The Navy and Marines are heavily dependent on the <a href='http://www.globalaircraft.org/planes/ch-53_sea_stallion.pl">CH-53 Sea Stallion and CH-46 Sea Knight, both old designs that also represent aging airframes.

Helicopters are much more sensitive to the effects of aging than fixed wing aircraft. As helicopters age, they require more maintenance and tend to have higher accident rates. When considering the V-22, keep in mind the accident rates of the vehicles they're intended to replace. One of my biggest concerns with the V-22 is its high pressure hydraulic system (reportedly 6000 PSI), something that could be vulnerable to combat damage. Pressures that high are used in those neat water jet machines that can cut through thick metal.