If anyone considers an app like Bonsai, it's important to know that there would be a steep learning curve if you want to use it for a GTD-type app. It's basically a very flexible outliner that by itself on a Windows desktop is a very powerful program, that also was designed to sync to a very capable outliner on a Palm PDA.
In order to get GTD-type functionality, I had to create a customized outline that would do what I want it to do. I am probably going to do a writeup about it for Natara's next quarterly newsletter (if I have time). The filtering capability is very powerful, and I can set it to show "Next Actions", "Action", "On Hold", ... or whatever other category I want, and it has a keyword feature that enables multiple (or single, just assign one) contexts.
I haven't spent much time on Omni Outliner because I am relatively new to the Mac. I just bought my Mac Book in November! It's very likely that Omni Outliner could also be used to implement GTD, but I don't know for sure. I am guessing that Bonsai is a little more powerful but that is a guess based only on the fact that Bonsai is one of the very rare superbly implemented applications.
Unless one wants to go through a steep learning curve though, I don't recommend it over a GTD app like Omni Focus. Also, if you are already on the Mac, I certainly don't recommend learning a PC program! (I am a Mac convert already).
If you do use a Windows PC though, perhaps take a look at Todumo. It certainly is no competition for Omni Focus but rather a minimalistic but very intuitive and powerful way to do the GTD methology. It might work well for things like a daily work to do list if you have a Windows computer at work.
When some kind of multi-context, keyword, or tag method is implemented in Omni Focus, then I will seriously consider it again as my main GTD-type app, and would even give up the sync'ing to the Palm PDA that Bonsai offers because the Mac Book is very portable. Most GTD-type apps do enable multiple tagging -- even the minimalistic Tudumo.
Tonight I tried iGTD and iGTD2 (which is in alpha) which do a good job with tags. However, one of the chief benefits of using an Omni Group app, IMO, is that Omni Group has development and support resources that the other Mac GTD-type apps don't have. iGTD2, which looks great, doesn't seem to have had an update in over a month. I would rather have a great implementation of Omni Focus from Omni Group.