The part that makes the IBDII great (more sensors) is also what makes it less than great (more sensors). If you can afford a good scan tool (around $125), OBDII is very nice. The cheap scan tools don't give you all the diagnostic info (especially the live feeds from the sensors). Because it has more sensors, it's superior at helping to diagnose problems (and does a decent job of self-diagnosing).
The downside is more sensors = more things that can break... and you need the scan tool to take advantage of the diagnostic capabilities. I was hesitant to shell out the $$$ for a good scan tool, but it's since been one of my favorite purchases. It doesn't always help me instantly know what the problem is, BUT it usually at least eliminates a lot of possibilities: a HUGE time saver.
As an added bonus, you can also use the live-scan to teach yourself how to do hypermileing (get the best gas mileage possible). You need a live-scan tool to do this. Google "hypermiling" if you want to know more, as it's really outside the scope of this thread.