Mitchell's is great! I am a huge East Coast Custard fan but it's hard to beat the convenience of Mitchell's when it's only a 5 minute drive from my casa.
Mitchell's does have some very good ice cream, it isn't amazing but it is worth it to me. Their vanilla bean malt milk shake is one of my favorite things there.
They also have a pretty good variety of ice cream flavors so you could easily try a new one each day for a couple weeks.
If ECC was next door to Mitchell's I'd probably go to ECC 4 times and then every 5th time I'd go to Mitchell's. It is a great place and I love it, but it's just not spectacular.