My DS had one just before he turned two earlier this year. He had it to see if he was refluxing into his lungs and if this was the cause of his repeated chest infections. It was.
As Ali said, having it put in was the worst bit, he found that really horrible and was very distressed by it. Once he got over that, he was ok as long as it wasn't moved, so if he caught it on anything that upset him.
We could take him out in the pram and did. I would also say, make sure you take some food with you that your DS likes, as DS wasn't overly happy eating, especially his lunch which was due just after the tube was put in. I think its important to get them to eat because, for DS at least, that triggers the reflux, so getting him to eat was important in terms of getting the information we needed.
The way it works is that it records the PH for 24 hours, so it records the number of episodes of reflux, how long each one is and how severe. They then take it away and analyse it. DS had already had an upper GI xray done which had showed reflux, but I understand that the pH study is good because a 24 hour snapshot is very likely to show up reflux if it is happening. DS's results showed moderate reflux, so he had lots of episodes but they weren't that long. The result, combined with the xray result and also the result of a bronchoscopy, all pointed to reflux and since he has been on the medicine he's been a different child.
Overall, try not to worry. It wasn't a pleasant day but it was very worthwhile and gave us results that have really made a massive difference to DS.
If you have any other questions, let me know.