I had to add my input since I do ultrasounds for a living. Usually around 17 weeks is when we can start to get an accurate look at the gender. However, sometimes we aren't able to see it clearly that early. It can depend on several things. Baby needs to cooperate and be in a good position. We have a much better chance of being able to tell if baby has its bottom up towards us and legs open than if baby is sitting with its bottom down and legs crossed. Also, if the mom is large then it can be hard to get clear images of anything.
I do have to add that determining gender of a girl based only on the fact that you don't see definate boy parts is a perfect example of why people get the gender wrong. There are certain things we look for either way. The position of the baby can effect what you see.
It also depends on the skill and training of the person doing the exam. Ultrasound is not very regulated on how much traning someone has to have to do it. I think the keepsake places are great because it is so neat to see your unborn baby, especially in 3d. But often they don't have much training so the exams are only for fun. Don't be affraid to ask the tech if they are registered with the ARDMS. In the ultrasound world that is the equivalent to a lawyer taking the bar exam or a dr taking his medical exams. Feel free to ask if they went to an accredited ultrasound program or just a weekend course.
As far as having an ultrasound goes, there are no known harmful effects. Does that mean there are some we don't know about? Time will tell. It is being used a lot more than it was in the past. Although now technology has improved so much that we are using less power and it doesn't take as long as it used to take to aquire the images. And the images are much better now. It's a personal decision to make about how many ultrasounds you feel comfortable getting. But I know lots of ultrasound techs, doctors, and nurses that use their connections to get scanned all the time. I used to scan myself all the time when I was pregnant and my ob dr was fine with it. But I will say that it is common for a woman with a high risk pregnancy to get many ultrasounds. But again, it is a personal decision that the expecting parents have to make with their doctor.