My dad made me take one when I was a kid and I did really well, but even then I thought it was stupid. The first issue is addressed in this blog post. The questions just aren't good enough. This is nobody's fault really, because it is very difficult to come up with a set of hard questions that measure your intellect and don't require lots of domain specific knowledge. It's either within a certain domain. If everyone taking the test was a Mathematician for example, the questions could be more clearly defined.
The second issue is that intelligence is multi-dimensional yet people try to measure it on a one dimensional scale. What's worse, the dimensions of intellect are somewhat elusive as well, so you can't just say something like IQ is the sum of values on each axis. Personally, I think that people that are high achievers in any field (business, arts, athletics, leadership, politics, science) are intelligent in some way. But when you look at all those people, there's no common denominator, no discernable pattern for intelligence.