Like others, I too have been frustrated (dumbfounded?) that there is not a better email client on the Android platform. Well, I guess Google's philosophy is to have an open platform and then let those apps (even a core app like email, apparently) be developed by third parties.
Another thing MailDroid has that K-9 doesn't is a folder tree view. K-9 just has a long single list, for example:
INBOX.Projects.Project1
INBOX.Projects.Project2
INBOX.Projects.Project3
While MailDroid is like this:
INBOX -> Projects -> Project1
-------------------> Project2
-------------------> Project3
If you have a lot of nested folders (I have over 100) then a single long list is almost unusable. There were some other things that MailDroid did that K-9 didn't also (and that I can't recall at the moment), but for me the folder tree view thing was the most significant. Some review of the K-9 development forum showed that the folder tree view issue had been discussed / requested for almost two years (as of this writing) and still was only listed as a "low priority enhancement."
Also, the developer (joelangley) is very active in responding to requests and developing the software. Take a look at the discussion thread for his software on SlideME, you can clearly see that he replies quickly!
Also, some have mentioned that $20 is a steep price for an app (I believe it's actually only $18). Personally, I don't understand this logic. How much have we spent on these Android smart phones? My Galaxy Indulge on MetroPCS was $400 (unsubsidized price)! I know others have spent hundreds on their phones as well. And without a proper email client, our wonderful Android devices would not be smart phones, they would just be phones! Personally, I am glad that there is someone out there with the talent to know how to program such an application, because I sure don't! In my view, $20 is cheap for this urgently needed functionality! So let's reward this guy for his work, and make sure it's worth his while to continue development going forward! Of course, if you don't want to / can't afford the $20, there is always the free (ad supported) version.