As a doula, I often tell my clients that I think the value of writing a birth plan comes in *writing* the birth plan more than in *having* one. As far as how they're treated in the hospital, I've seen some nurses pay a lot of attention to them, even reminding my clients of things like, "Your birth plan says you would like a mirror during pushing--would you like me to go get one?" Or whatever. I've seen others who don't read them.
I personally don't put a ton of stock in birth plans. I think it's good to consider your options (which is why, I suppose, *making* one can be valuable), but I guess I personally feel like there's a lot to be said for NOT "planning" your birth too precisely. Better, in my opinion, to be well aware of your options and to make your preferences known as your labor and birth unfold.
If you are going to make one, though, my biggest piece of advice is to keep it as short and simple as you can. A lot of those online programs prompt you to answer a million questions, and then render you this 4-page "Birth Plan" that you can be assured everyone isn't going to read. If you want, do the online birth plan, then copy it to a Word (or similar) document, and pare it down considerably before printing it. Begin by omitting things that are not standard practice at your birthplace anyway (for instance, it's probably not going to be necessary to tell the staff that you do not wish to be shaved or given an enema upon arrival at the hospital--but most of these birth plan prompts seem to ask that...in fact, I've often wondered what would happen if you wrote into your birth plan that you *did* want them to shave you and give you an enema when you arrive, but that's another story

). Next, go through and select the few things that are most important to you. Saying things like, "I want to be able to move around freely during my labor" is really unnecessary--your ability to move around freely during your labor is going to be a result of other decisions (having an IV, allowing continuous fetal monitoring, having pitocin, or narcotics, or an epidural...you get the idea). So rather than telling someone you want to be able to move around freely, perhaps you want to decline an IV upon admission to the hospital and request intermittent fetal monitoring. Which you might put in your birth plan, I suppose--or you might just inform them of that decision when the time comes.
It's wise to talk with your doctor/take a tour/ et cetera to become informed of what the protocols are at that hospital. That way, you can be prepared for what things you will have to specifically request or maybe push for, which things will just happen automatically the way you prefer, and which things may just not be an option for you at that particular birthplace.