I love them in some ways, and hate them in others.
A couple of sessions ago when my party was travelling through the Underdark I rolled up a couple of random encounters. I gave the players my 2 sets of Underdark tiles and told them to make up an encounter area.
They had a great time just creating the area the fight would be in. They enjoyed it so much that I through in another random encounter just so they could set up another one. That worked really well. While they were all sorting out the area it gave me time to just go over the stats of what they were fighting one more time.
About a year or so ago I used the Wilderness set to make another couple of random encounters. That also worked really well. The tiles really do help to set the scene more than your ordinary battlemat.
On the flip side to this, I am running the Shackled City AP. It has a lot of dungeons in it. Some of them are quite huge but there are a number of smaller ones as well. Despite owning 2 sets of each of the first 6 or 7 dungeons I still struggled to be able to recreate the dungeons. The main problem was that a number of the rooms were so big that I would have had to made them up out of 10 or more pieces. Another issue was that many of the rooms weren't square or rectangular. The tiles don't tend to work to well with odd room shapes.
So despite me wanting to use the Dungeon Tiles I found that it was going to be too much effort to make it worthwhile. On top of that, I've also found that when I do use them everyone has to be careful not to knock the tiles around. They move around a little too easily.
So in my experience I have found that the Dungeon Tiles work really well for random encounters, but not as well for dungeons.
Olaf the Stout