For those specifically looking to improve VO2max, the PowerLung gizmo appears to be effective. I have not used it, but I do know various coaches and athletes that do, and it's been popular with them. The maker also supplies some data on "Respiratory Muscle Training" to support their claims here:

http://www.powerlung.com/region/us/i...tion/research/

It appears this gizmo has benefit for those looking to improve VO2max. The studies make for interesting reading