Crutchfield is one of the best site for car audio and have been around for along time. I would definitely trust them and have no problems with doing business with them. Polk is a great brand of audio to go with. They are on of those brands that in my opinion you cant really go wrong with them. I have hads Polks in the past and I have no complaints. The infinitys are very good also and my personal favorite. You wont go wrong either way.
they have high prices though
i have polk 4x6's in the front and 6x9's in the rear and they are great. crutchfield is real good to buy from but they do have high prices.
I havent personally dealt with Crutchfield but my friends have. The slight priciness does get you some amazing customer service. I usually go eBay but if something goes wrong I'm effed but it is cheaper so you gotta decide if you want that kinda situation.
Crutchfield is a great site. Alot of useful information, how to's, and a very friend staff. I just recently bought some polk subs off them because they were have a buy one get on free sale. Usally their stuff is expensive but they do have good deals going on, you just gotta keep your eye out on them.