I was actively involved in the PTO at my daughter's school her first two years there (this is her 4th year), and I actually was the PTO secretary for a time. While I was on PTO, parental participation was very low, so I did a survey to determine why people did not participate in PTO, and what we could do better. Resoundingly the answer was that other parents thought the PTO board was "cliquish and unwelcoming," and by and large, this was true - I experienced it first hand, and I resigned as secretary.
At first I was all "gung ho" to make a difference, to be involved, because I strongly believe that a child's experience at school and his/her relationships with staff and peers are enhanced when parents are actively involved.
What I quickly realized was that, at least at my daughter's school, the PTO had been run the same way for many years, and they were not interested in new ideas. They put on the SAME fundraisers every year, and the SAME activities for the kids every year. The board had been comprised of the SAME individuals for many years, and therefore adding me to the board was just essentially adding another pair of hands. Well, I don't work well like that - I have opinions and ideas, and I want my voice to be heard. The other board members were very resistant to change under the guise of "this is how it's always been done."
The other moms on the PTO were also SAHM's, and I found I really had nothing in common with them - the PTO was their job, whereas I was trying to fit volunteering around my full-time job and active church involvement.
So, it didn't work out for me, and I don't volunteer for much anymore. I have a good rapport with the principal of my daughter's school, as well as with my daughter's teachers, so I am still involved in non-PTO events, such as chaperoning field trips etc.
I think every parent should find some way to be a face at their child's school - PTO isn't the only avenue for that. And I'm sure every PTO is not created equally, but I realized it wasn't a good fit for me. Another respondent said there are just those made to volunteer and those that are not. In my experience with PTO, there are those made to follow blindly, and there are those that are not. I think the key to a good PTO is to make parents feel welcome, and to make them feel like their opinions matter. The PTO board should give others a say in activities, as opposed to planning the activities and saying "this is what we're going to do - this is what we need you to do." The questions should really be "what would you like to do," and "how would you like to help?"