I've been using it for about 3 years. I'm on the Standard plan (middle tier). It's $4.20/mo. per user when prepaying for 3 years and ranges up to $5.40/mo. for monthly billing.
Not sure if there's a (practical) limit on domains or aliases, but I have 7 domains and a few aliases plus a wildcard. Includes 30GB of storage per user.
I like their integration with 1Password to generate email aliases on the fly. Great for signing up for trials, then deleting the alias if the trial doesn't work out. Saves you getting spammed for life just because you wanted to check something out, and hides your domain to keep things on the down low.