tbqh, it's really not even that good! It's the closest thing to macrobrewing in microbrewing style in the world. That is, it's the largest, most widespread microbrew in the world. If you're really in love with it, try their seasonal stuff (and only if you can get it fresh!). Chico Estates is awesome when it's fresh, but if you like the bitter more than the nutty, Torpedo is the better. They also do an alright Celebration every winter.
If you really like Sierra, though, find a place nearby selling Bell's Two-Hearted Ale. It's the best beer in style you'll be able to find. Though, if you want to spend a fortune, Three Floyds is truly best in style, but kind of an experience to try. I got some for Christmas a few years back and it was delightful.
Basically, if you want to know about IPA at all, I've had basically every top-rated one in the states (it's my style of choice), and there are most def. worlds better than Sierra.