I'll add my $.02. I like to travel to remote areas but need cell phone and internet access for work so extending my range is really important to me. Right now I'm using sprint (with roaming!) for my cell phone service and verizon for my laptop data card.
An antenna is simply a must have for anyone that travels outside of the city. They make a big difference and there are plenty of places I'd have no signal without my antenna but good signal with it. I have both of these antennas
http://wilsonelectronics.com/ViewProductB.php?ID=5
http://wilsonelectronics.com/ViewProductB.php?ID=6
I've been running the bigger one for over a year now and it has been invaluable, I've had cell phone reception where without it I wouldn't have, and I've had it take me from a poor signal to a good signal. I've only had the short one for a little while but when checking it on my verizon data card I saw similar db gains with both antennas.
For amplifiers, well you have to keep in mind they only amplify the signal your phone is getting so no signal, no amplification. I'm not using a wilson amplifier I'm using a different brand, I use a direct connect setup so the amplifier is connected inline with the antenna and then connected to the antenna port on my phone or data card. From what I understand the direct connect amplifiers allow your phone to more accurately manage the power output from the amplifier than the ones that are not direct connect setups.
I have had some inconsistency with my amplifier, I've had problems where it wouldn't let my verizon card connect to the network when it's plugged in but if i connected without the amplifier and then hooked it up to the card it would keep the connection and increase my signal. I've also had people tell me I was coming across very loud/distorted on occasion. Keep in mind this is not with a wilson amp, I'd really like to try a wilson amplifier to be able to compare it someday. The biggest negative with the amp I have is that the antenna doesn't work with the amp powered off, I find that very frustrating as it means if I want to go from using just the antenna to deciding i need to use the amp, i have to disconnect the antenna and phone adapter and then connect those to the amp, kind of a pain, I was hoping I'd just be able to leave the amp hooked up and with it powered off just be using the antenna and then if I needed a boost turn the amp on. That said it has definitely made a difference at several campsites for me and I'm glad I have it.
However by far the best bang for your buck is just the antenna, the antenna and phone adaptor should run $30 or less, it will increase your coverage noticeably where as the amp is going to be $200+ and will only increase the signal, it can't help you out if you don't have a signal at all.