They made the center channel woofers smaller and it doesn't sound as good. The did away with the 5 1/4" bookshelf speakers and now only offer the small, 4" model. The grilles on these are very cheap. Thin fabric with small plastic posts that are very easy to break. Just like anything else now days, Pioneer cut some corners with a new model line. Speakers still sound about the same, at least the ones that weren't changed, but now the towers are sold individually, where as the previous towers were sold as a pair. Just another way to raise the price. Thumbs down on the new line.
I see. Not a lot that concerns me there. I'm running a 2.1 system so the center channel or bookshelf speakers are not an issue. Grill material doesn't matter to me (and I think the new ones look nicer anyway). Most of the reviews mention that the FS52 is a better performer than the FS51 (specifically the highs).
I don't know what the FS51's sold for originally, but I got my FS52's for $200 total and I'm more than pleased with them at that pricepoint.