My apologies, I just noticed that you had initiated the conversation about Mixmax in another thread.
Yes, I like all these features that you mentioned, though I can't see yet how "Seeing who opened" can be that helpful. I mean, if someone doesn't open your email you won't send them another e-mail saying something like "Hey, I noticed that you didn't open my email...", right? But rather you send them another email stating that on mm/dd you sent a proposal and would appreciate receiving a response from them, isn't it?
I noticed that my Gmail crashes a couple of times per day and I believe it is because of Mixmax, did that happened to you? Also, Mixmax is preventing other add-ons from working properly or entirely.
I attended a webinar organized by Streak and left it with the exact feeling, too complicated and it doesn't seem to fit for freelance translators.