We have purchased over 40 vehicles. Over the last few years, I have used the Internet many times to get quotes from different dealer's Internet departments before even talking to a person at a dealership. It has helped me to narrow my search down to the dealers offering the best overall deals. Once I have them, I usually call the various Internet Departments to get answers to any questions I may have.
That said, I also then take my printouts to the one local dealer to see what they can do. Usually the Internet department at out of area dealers offer a better deal. These are usually larger volume dealers in N. CO or Denver area. I may have to drive 75 or 100 miles, but it has sometimes saved me $2,000 out the door(including trade).
The dealers also give me a range quote on my trade based on its condition. I have never received less than the bottom of the range they quoted me on the net/phone. Have purchased and traded several vehicles this way.
Any dealer who will not give us a quote without coming in to talk with them, we cross off our list. We are not sheep, but people who won't work with us over the net and/or phone are definitely wolves. On the plus side, We have found several reputable dealers what we have done repeat business with, even used the same Internet salesperson for three different purchases over a five year period.
The local dealer on our most recent purchase beat the dealers down in Colorado by about $500 bucks on the exact model we were looking for.
The local also gave us $1,000 more for our trade, which was unusual.
All the negotiation was done in advance of going in to the dealer and of course was subject to our test drive and approval of the vehicle. We spoke to the finance manager in advance and told them we did not want an extended warranty or rust and dust and if they hassled us we would walk. There was no pressure.
As far as test drives, sit down in advance and make a list of all the features you want/need. Prioritize them. Drive the vehicles you are considering under the conditions you plan on using them. Check all the items on your list against the features in the vehicle. Make sure you try all the features out thoroughly. You may need to take a couple of test drives. Make sure the dealer understands that you will be doing this. If they refuse to allow you to do any more than a cruise around the block... cross them off your list.
Getting back to the purchase, even though we went with the local dealer, the net and interfacing with other dealers still paid off as we knew what the model we wanted was going for and what our trade was bringing.
Edmunds and NADA guides on the net can also give you an idea as to new and used pricing, rebates, incentives, etc.