I'm a happy user of ImprovMX.com - used it for free for a long time, now paying some 30$/year. Delivery is pretty much instantaneous, and I had only very few mails not being delivered (and there was a legitimate issue on the sender side, so ImprovMX was rejecting it).

But I'm not really satisfied with the sending side of things - my plan doesn't include the feature, and sending via Gmail is enough most of the time, but setting up each alias manually in Gmail is quite annoying.