Our party just ran into an enemy using Spike Stones. Its usefulness is dependant on level. As a 4th level spell it won't show up till CharacterLevel 7, but by level 11-12 it stops being all that usefull. Our DM plays that it is an area denial spell and that you can stop at any time. Our party ranges from 12-15 and overall it was more of an anoyance than a threat. Our dispels were concentrated on more imediate threats and our party pretty much sucked it up and ran through to get to the bad guy. We had to go about 20 feet to engage and that comes to 4d8 reflex half or about 12 damage on average. The movement halved was not that big of a deal since a cure spell takes care of it and the cleric with the mass cures took care of everyone in one shot.