The best feature is that it can deliver emails to every single address in your database, not just ones you have verified! This could be helpful if someone has changed their mail server or addresses but still needs some of those old records sent out from an older campaign/list etc..
I would like more features than what they currently offer (esp when compared with other services), however there are great tools available now so this may change over time as competition heats up even further!! You need at least 3 months worth data before using any service since its all stored locally until then meaning downtime isn't too much trouble either unlike say gmail where everything goes down once per month which causes lots issues during busy times - especially considering how many people use Gmail these days!