I had a little princess spirea in my front garden for years and every year it reseeded. I had little babies popping up every year. I dug them and gave away a ton and also moved some to other places in the yard.

My neighbour's bridal wreath spirea hangs over my fence and it has apparently dropped seeds as I have had two bridal wreaths pop up in my yard.

interesting.. now I am not sure they will make the best shrubs, any other comments on them when full grown?

I think spireas are great shrubs. There are so many different ones to choose from, with lots of newer varieties. Even with all of them, though, I really love the good old fashioned bridal wreath. Definitely worth having.