Anyone have any experience using any of Sparkfun's IMU devices?
They can be found here (I'm looking at the 9-axis ones):
I've seen the tutorial found here:
But I wanted to get some feedback from people who had actually used them and had an alright time with it. I've never used the I2C interface before for example, and wondered if it was a nuisance or super awesome.
Thanks in advance! 
The I2C bus is slow. I read about people with small RC helicopters that needed a higher sample rate. For normal sensors, and for an IMU board with a sample rate of 100Hz, the I2C is a good bus to use.
An IMU board is nothing else than a bunch of sensors and perhaps a voltage regulator and maybe a I2C level shifter.
Most sensors are 3.3V and most Arduino board are 5V. So you might need a level shifter.
Some IMU boards use old outdated sensors, just because there are good libraries for it.
Don't buy an IMU board with a seperate ITG-3200, because the MPU-6050 is newer, cheaper and better.
The support for the MPU-9150 is somewhat limited, a MPU-6050 + HMC5883L is just as good. If you don't want two seperate modules (MPU-6050+HMC5883L), grab some hot glue and stick them together 
What do you need ?
Acceleration sensor + gyro + magnetomer (compass) + GPS (location) + baromic pressure (altitude) + even more ?
Then you should buy a board with modern sensors that have good libraries for it.
My suggestion is to buy a 3 dollar MPU-6050 board and try to make it work. Turn it around and see how the values change. Learn about filtering, and Kalman filter. In my opinion, the Arduino is all about prototyping and learning along the way.
Before buying something, read about IMU and Kalman filter in this guide : Guide to gyro and accelerometer with Arduino including Kalman filtering - Sensors - Arduino Forum