I have a buggy board and used it quite often in the past. I used it with a Maclaren Free Spirit. I loved that I could still recline the stroller most of the way for my DS to sleep while DD rode on back. It's a good size area to stand on, and she was able to ride on it well at only about 28 months old. I did find it a bit of a pain to have to keep taking it off every time I folded the stroller.
But when we went to WDW last year, we bought a Peg Pliko P3. Although I could no longer recline the stroller while DD rode on back, it was much more convenient to only have the one piece. The only downside is that I think a child really needs to be over age 3 to be able to balance good on those little steps. The P3 steps are a little easier to stand on than the older Pliko models.
I would say get the buggy or kiddy board (depends on what type of stroller you have) only if you really love your single stroller o rif your "rider" is under 3. If not, you may want to consider a Pliko.