I use SimpleMU for most the reasons mentioned above - spawns handled a way I like, auto spell check. I also love how easily it runs off a flash drive.
Literally the only thing that I want that SimpleMU doesn't do is to handle https links as something other than email.
I've tried Potato, but I had real issues with noticing when there was activity in my spawns. It handles notifications in a really weird way that I just could not get the hang of - and I tried!
It also has some minor things, like not auto-spacing logs, that I don't like, but the fact that I kept consistently missing things in my spawns was what killed it for me.
I'd love to see someone code a newer, better option that takes on some of the things people like about old programs and some of the stuff available for Mac. But there isn't one yet, SimpleMU still works seamlessly for me, and I've learned (annoyingly) to c/p https links.