There are some really tricky things that you can do with the Geomancer. The summon stones gave me so many troubles when my group first used them as well. I was entirely unaware that the stones were capable of their own movement activation until about a month ago, proves how much I can pay attention to some things. However, there are a few things about the stones that work amazingly and at the same time really limit you. It is great that you can summon a stone around a corner (given proper space counting) then attack with the stone. The thing being at that point, the stone doesn't get the bonus surge that it grants to the Geomancer and you trade off one of your own attacks or other actions to summon said stone. There is the is a skill that you can get for 1 xp, can't remember it off hand, but it allows you to make a blast attack with your stone as the target entirely ignoring range and LoS restrictions that destroys the stone but nails everything around it. The advantage to this is that you gain your surge for the surge to your attack because the monster is adjacent to your stone (you only gain one surge based on following the rules for "Do Shadow Dragon Abilities Stack?") and usually can then kill a whole monster group in one go.
A good tactic to use is to be the first player to go, spend your heroic feat, fatigue move 5 spaces, summon your stone, then blast attack with it. You can usually open the road for your team while having a fully cleared stamina card. It also works well if you pair it with a double move for a total of 16 spaces of movement. Agreed this does put you at a severe disadvantage because you are left on your own, but hopefully you moved where the OL can't reach you and attack in the same turn. When you start off next, you summon up a stone and block off passage from the OL while making your attacks. Again, another disadvantage is how it will block your LoS, but that has come in handy more times than I would care to count. Moving them in front of doors before opening them is really clutch, too. Take, for example, the Castle Daerion quest or Cardinal's Plight quest. You place the stones so that it forms a wall that the OL has to break through before he/she can actually attack the heros. In the First Blood quest, my OL had the two Ettin's stay in the Fire Pit, staggered so that the Master was in the back. I just summoned the stone behind the front Ettin, attacked it, and then used the stone attack on the Master.
I wish you could make attacks with the stones at every activation, but I honestly understand the balance problem with them being able to do so. I now understand that I have entirely changed the topic of this discussion to a strategy one instead of the understanding of how the stones themselves work, but I guess on the flip side it is still keeping in character with the topic.