Depends on what the placement is and where you want to go with it -- it can be really good experience if you're going for a science degree, introduction to proper research technique and presentation etc, looks really good on your PS and so on. At interviews they always want to talk about what you did and what you learned.
If you're looking to into medicine or dentistry, shadowing or patient/caring experience is probably more useful, and if the placements vary in how interesting/useful they are.