First, if you are trying to decide between iPhone 4 and 4S based on price, go ahead and get the 4. I have no desire to use Siri. My iPhone does everything that I need it to do and really Siri is the only difference between the two phones. If you buy an iPhone 4, it will have the new operating system. I have not upgraded to it yet.
I've had my iPhone 4 for almost a year now and I purchased mine directly from AT&T. I have had a great experience with my phone. Before I switched to iPhone, I was a dedicated BlackBerry user. I have owned 4 BBs and I was really anti-Apple. I finally got tired of the lack of apps on BlackBerry and the outdated email interface and decided to switch. When I went to the store, the girl initially talked me into a Windows smartphone. That phone started freezing up on the very first day that I purchased it and some of the apps were unresponsive. I immediately took it back and got the iPhone 4 and I'm glad that I did.
I've never had any problems with phone reception or slow loading speeds. I have gotten a few dropped calls, but I'm pretty sure that is due to AT&T's issues. I use this phone for EVERYTHING from navigation to calls and texts to keeping personal lists. I use apps related to health, my bills, banking, discussion forums, beauty, and more. The steepest curve was getting used to limited data usage. You can only use so much data per month. With other carriers and phones, I had unlimited data usage. During the months that I had access to Wi-Fi at home, school, or work, I never went over because I could use a Wi-Fi network for most of the day. However, during the times that I tried to stick only to 3G, I went over my data usage. The only thing that really caused me to go over my data usage (twice) was streaming music and video. But, like I said, it's usually not a problem.
The feature that BB has over iPhone is easier texting. It can take a while to get used to iPhone texting and the lack of emoticons. There are emoticon apps, but only iPhone users can see the images. My iPhone email is super-fast and really convenient. I have 5 email accounts connected to my phone. I only load emails on accounts that I am using, if that makes sense - it doesn't load and store all of your emails unless you set it up that way. That would take up too much storage space on a 16GB.
I've had my iPhone for about a year and overall, I am very happy with it. If you do make a purchase, don't forget to protect your investment with the right case and a screen protector.