Yes, I also use the Sovereign Stone magic system. It ported real well into my world (I call Elemental Mages - Druids and Void Mages - Witches), due to the way I wanted to use the four elements and the Void (which I call the Blight in my world).
I also use the classes in the Sovereign Stone campaign setting to replace the magic-using ones from the PHB.
I do not have complaints yet, but none of my players have taken the plunge yet (out of the 10 PCs in three campaigns, I have 2 Barbarians, 4 Fighters, 1 Monk, 1 Rogue, 1 Archer from Sovereign Stone, and 1 Mounted Warrior from Sovereign Stone). Each party does have a Druid NPC with it. I am very fond of having NPCs join the adventurers, as I usually run small groups of 3 or less PCs. In the 6 PC group I am only using the Druid so I can continue testing the magic system (I figure that if I keep using it, I will be able to find and fix any problems before the PCs start playing a spellcaster).
Overall, I really like it (especially the chapter in Codex Mysterium on creating your own spells - it can be a little hard to understand or create the exact spell you want, but it is still very useful).
Having the spellcasting process use a d20 plus Spellcasting Bonus works real well with designing items/areas/or effects that enhance or diminish casting power, because like combat/skill/save type modifiers I can have modifiers to the Spellcasting Bonus.
I have been interested in how many others may use this magic system, so I will have to keep my eye on this thread.
Also, if anyone has created their own house rules, spells, items, creatures, etc. using this system and would like to share them, I would be very interested. I do not know if you want to post them or send via e-mail, but I am receptive to either forum.
I may post some of my spells (ok, at the moment I have only created an Obscuring Mist-style spell) as I create them.