My players are now ecl 13, and Grim n Gritty is holding strong. My experiences summed up? It plays exactly like the normal rules, EXCEPT!
1) The CR system is TOTALLY messed up. You can't even just say everything is strong. No no no, not at all. Some things are heavily nerfed by GnG, some things extremely buffed, some things about the same. There is no systemic bias to be corrected. Everything is now practically random in terms of power. Throw out everything you used to know, and learn how it works anew.
2) Tactics need to be reworked. And it's not just in terms of ambushes or who gets the first hit in fights. What happens is you can no longer rely on having spellcasters/archers in the back, melee guys in the front, and keep hitting each other until someone falls down. You have to cast spells intellegently, move around constantly, and really work for those flanking bonuses. Buffs and anti-buffs become the hero spells of the day. You pay attention to cover and you USE IT. Attacks of opportunity are feared. Damage over time effects are crucial.
3) Things feel more active. You roll for defense, and that's nice. Things also feel more real. AC is split between "Beefyness" and "Agility". Some characters almost never get hit, but when they DO get hit, they hurt. Like you would imagine. Other characters get hit often, but when they do get hit, they don't feel it. And also like you'd imagine, those that soak a lot are susceptable to the fast agile guys who go and find openings and chinks in the armor (higher chance for a crit). The cinematic feel, instead of going away, in my experience is even heightened.
The biggest change though, is the CR thingy. Ogres aren't CR3 anymore. Not by a longshot. Dragons in GnG get their CR raised by about 5 in my estimation. I've found that a Nymph is far, FAR weaker than before if you go in with the intent to kill.