I just want to share some thoughts...
I've used a program called Switch5 for ages, since I had my Zire 21, it's free and has a feature much like the browsers' "Back" function - it takes you to the app you were using before (excluding Launcher, which it ignores). You can access this feature in several ways, like hard buttons or sweeps from Home to find, etc.
Currently its development is stopped and that really hurts users of the newer Palms with Virtual Graffiti or no Grafitti... the only way to access it is using a hard button. Even so, I use it on my TX: the first Hard button (Home) is for this feature, and I use HomeWithFind (MyKbd would also do the trick) to access Home from the Status bar.
I feel that once you get used to thinking in terms of "Back" when navigating through apps, it becomes an essential feature of your Palm. It's just so much simpler to use a Palm when you can switch back and forth between two apps effortlessly.
I think about 60% of the usefulness of DA's and OnTop (but definitely not 100%) can be achieved with this Back feature, because when you're in the middle of something and switch to your Calculator or Contacts app for a quick reference, you just press Back and go on with your original activity.
That advantage is that technically it's a much simpler thing, no memory issues, no compatibility between apps issues, and it's very very useful.
I just felt I should share this with everyone...
(Truth is I gave this app to a friend of mine and he just deleted it after a month, never got used to it...
I'm starting to wonder if I'm the only one who likes it).