So far I like WYSIJA/Mail poet a lot.
Some things I wish you could do/were easier:
Making sign up forms nice, adding color, changing fonts and font sizes, adding pictures, etc. I think there is a plug-in that you can use to do this, but I haven't played around with it yet.
I have found the stats (free version) to be inaccurate. I have only had a couple people unsubscribe but when that happens it never shows up. Also have seen newsletters with more clicks than I sent it out to and there was only one link.
I would love to see a text only option.
You can do an auto responder, which is wonderful. Getting people on different lists is simple, and if they join another list it doesn't duplicate an email.
I like that wp interface, though I kind of wonder if it slows down my site some.
Easy to use shortcodes and widgets. I tried php once and it didn't work, but I think that is just my inexperience.