MailJet is extremely easy to use and makes the process of generating mailings extremely simple. The UI for generating mailings is intuitive and simple, yet powerful and able to accomodate pretty much anything you'd want to put into an email. I use it generally for doing newsletters to a large group of about 5,000 subscribers.
The overall interface for managing Contacts lists and Campaigns is well done and understandable. Adding and removing subscribers from Contact lists is fairly straightforward and mostly great.
On the rare occasions when I've needed support, they've always been extremely fast and knowledge-able, and solved the issue in short order.
Three things about Contact lists are a bit annoying:
1) There doesn't seem to be any way to automatically remove Unsubscribed Contacts: you have to download them manually, then upload them to remove them.
2) Same thing with Bounced or other Contacts that you discover from the post-Campaign analysis: there's no button to say "Remove these from my Contact List": you must download them, then remove them through the removal interface
3) Errors from uploading new Contacts is a little clunky when there are errors: again it generates a file that you have to download and look at separately. For most errors (duplicate address, typos, invalid format, etc) in the little uploads that I do as part of expanding the list, I'd just like to see the darned things immediately on the screen so that I could either ignore them (i.e. dups) or fix them (typos & invalid format). Only if the errors go beyond some threshold (say more than 10, and it could be user-settable), should it do the file-generate-and-download thing.