In any case, I think Haskell will need reliable package management on at least OS X and Linux before most developers consider it a serious choice for real projects.
Previously, I had more issues with conflicting version constraints which I think is more of an issue with library authors and not necessarily cabal itself.
Edit: At least, I can't get the new version of Cabal/Cabal Install working on my Mac.
Have you sought help from the #haskell IRC channel?
And I agree that sandboxes are great!
My biggest issue is that I don't have the expertise to debug an obscure Haskell compilation error. I won't develop that expertise unless I can use Haskell over the long term on real projects. I can't do that unless libraries are available. So it's a chicken and egg problem.
I think the same is true for many people who'd like to dive deeper into Haskell. If we can't initially lean on the work of expert package maintainers, we can't ever become Haskell experts ourselves. I believe the developer community could expand very quickly if this problem could be solved.