Here is a summary of their popularity
#1 They've been pretty much the king in the Midwest for a number of years. I remember 11+ years ago waiting outside the Urbandale Hy-Vee for the ability to buy one bomber(22oz) of their beer Psuedo Sue. The Urbandale Hy-Vee would usually get like 1 case (12) per wEek and it would be gone within minutes. Now you can find their 4 packs state wide at about every place that sells beer. Some places they even collect dust.
#2 They are in Decorah and the first time I went there their taproom was an old pizza hut but they had KILLER beer. They've since build a huge facility on the outside of Decorah. Really cool place.
#3 For a long time they were Iowa only. For awhile they contracted out to someone in Florida to brew their beer, which IMO was a complete disaster. I think this only lasted a couple of years. Since that time, their footprint has gotten rather large and people that get their beer in new states love it.
#4 They are known for two major things. IPA's and world class stouts. They hold lottery's for people to come and pay $200 for a couple of bottles of their stouts. People drive from all over the country to get these stouts and very few people actually get the privilege.
#5 Like other successful companies, after they have some success, people start to hate their success. Part of this may also have to do with the owner not being the most loved guy in the world. These people that hate them can usually be identified as saying something like "Why did you go to Decorah and visit Toppling when you could have spent the whole time at Pulpit Rock (Another excellent brewery with TG ties).
With all of that said, the craft beer business has changed a lot in recent years to become a lot more local. Places like Lua get more love, deservingly so, than the big whales like Toppling Goliath.