I've been using MailDroid (free version) now for a few months and have to say that it is by far the best mail app I have found, and I think prior to this I tried just about every free one there was.
The ads I don't even notice, so that's not a big deal to me. But I do love the fact that it more or less operates like the mail I was used to on BlackBerry. I can delete messages only on my device, or from the server and device which was something I could not live without. I'm not certain on how many email accounts you can use with it, however, I currently have four setup. Plus you can have the emails from all accounts in one Combined Inbox, and even add bookmarks if you'd like.
All this being said, I was having some gradual issues with the app crashing almost every time I used it, but this wasn't until the last two months or so. Then about a week ago there was an update, but unfortunately the issue only became worse. I did attempt to contact the developer who was very prompt and friendly in replying, but with my device being rooted his suggestions were of no real use.
So instead I contacted a friend (the one who walked me through rooting etc) and he suggested that I download and install ROM Manager, and run the Fix Permissions feature. This immediately fixed the issue with MailDroid crashing and I haven't had any problems since.
Overall I give MailDroid an A++
I highly recommend it to anyone looking for a hassle free way to manage several email accounts, and especially to those who are looking for mail similar to that on BlackBerry.