I feel that using Git submodules is somehow troublesome for my development workflow. I\'ve heard about Git subtree and Gitslave.
I currently use submodules for development and not just relating 3rd party libraries. There are some ways that you can make life easier with submodules, especially when they are the source of merge or rebase conflicts. Look to ls-tree to get the 2 commits involved on a conflict in the submodule. This is probably the most difficult part of submodules for people to deal with. For now scripting will make this much easier to work with. Future versions of Git should have better native support for dealing with them.
Hope this helps.