Upfront, I'm the co-owner and developer of ActiveCaptain. We sell a new type of amplifier that has already been discussed here. I'm going to try to stay away from "selling" anything here but I can give some significant information after working with almost 200 users of these devices.
First, we have an 18 part series about all aspects of using a mobile phone on your boat. It's all free and easy to access. The article about amplification is here:
ActiveCaptain - Going Mobile - Mobile Phones Series
This article gives some background information behind the issues of passive and active mobile phone amplification. There are a few different types. We've tested them all on a few boats in real situations over the last 5 years. In general, they work really well, except when they don't - and I'll explain why.
There are times when there just isn't enough signal to do anything with. The Pungo-Alligator Canal in North Carolina comes to mind. It's a 25 mile straight cut through the middle of nowhere. We couldn't find any mobile signal out there at all. It's a complete dead zone. There are also some situations where there is so much electrical noise, the amplification can't pull out the signal from the electromagnetic mess it is receiving.
In general and unless you have a metal superstructure, a traditional wireless amp (especially the Digital Antenna one mentioned earlier in the thread) will not provide adequate results. The problem is a feedback loop that gets created because fiberglass and wood don't provide enough separation between signals. More about that is explained in the article.
Cell Ranger is a new type of amplifier. It only amplifies half the signal - the part coming from the tower to your mobile phone. That's why is it less expensive. It turns out that for a variety of reasons, towers can pick up your tiny phone signals really well. Their huge antennas and expensive electronics can "hear" anything you send out. The problem is that your phone with it's tiny antenna and inexpensive electronics needs help to receive the signal coming from the tower. The output from the tower is limited by FCC regulations. Electronics at the phone can make up a lot of this loss.
There are still times when the problem is bi-directional - usually if you are really far away from the tower and there is something in-between. In those cases, an amp just isn't going to help. But boy, it's not easy to find those places these days.
For an example of electronic noise, here's what happened to someone who got a Cell Ranger and couldn't get it to work with his air card on the laptop. He was using the USB version of Cell Ranger which was plugged directly into the USB port on his laptop. The aircard was on the other side of the laptop. So between the two devices was the entire bulk of the laptop with a very large amount of electronic noise. When he could only get 1 bar on his cell phone, he got no connection with this setup. I had him move the Cell Ranger to the other side of the laptop with a small USB cable and guess what? Four bars.
We've been working with the manufacturer of Cell Ranger to make some changes to the product for marine use. The device is really designed for automobile use today and after providing it to a couple of hundred boaters, there are a few things that we've learned. They've responded and are close to providing us with the first units to test. If you need more bars today, Cell Ranger is fine. But if it's really something for spring, wait. The next version is more appropriate for on-the-water use and provides more flexibility and capabilities.