SaneBox is great for anybody who a) gets a lot of emails, and b) needs to respond quickly to email. For example, as a consultant, it's important that I quickly read and respond to my clients' email messages. SaneBox would be less useful for people with low volumes of email, or where there is little or no email from external senders.
I like that it's free, I like that it's open-source, and the fact that it allows IMAP lets me have this on all 3 laptops and it's synced. I would never consider using it as my primary email client, however; just too clunky and can't navigate around fast enough the way I do with Gmail. And not even close to having all the features I'd need that Gmail does: 30 second undo-sending, canned responses, easy-to-create signatures for each email address (I literally toggle over to Gmail, compose a message, and paste my sig into the body of Thunderbird emails, so clunky), all spam ends up in my inbox even after I've marked enough as spam that it should know, email labeling and filtering... Gmail is just way more advanced and feature-ful. But, Thunderbird does beat my ridiculous RoundCube webmail interface!
I use the Snooze and Spam features in Office 365 for another mailbox. I feel SaneBox's system of folders is easier to manage, plus it is available from anywhere I check my email. Office 365 snooze is not available through my Android email program and in [my opinion] it's aggravating not being able to organize email from my phone.