OK, so we just playtested the guns, not the gunslinger. If people don't like that, ignore this - but you shouldn't have to have a special class to make a new weapon useful.
Our game has been using guns for a while, and we have one character, a cleric of Gozreh, who uses them as his primary weapon in our Riddleport Pirate campaign. Up until last session, here's the gun rules we were using:
Pistol 250 gp 1d6 2d4 x3 50 ft. 3 lbs. P
Musket, short 500 gp 1d10 2d6 x3 100 ft. 8 lbs. P
Musket, long 750 gp 1d10 2d6 x3 150 ft. 10 lbs. P
Blunderbuss 500 gp 1d12 3d6 19-20/x2 15 ft. 8 lbs. B and P
Exploding dice of damage
Misfire on 1
Two full round actions to load
Full rules if you really care about more details
We haven't been completely happy with the results; he misses a lot and deals very low damage amounts when he does; he often just uses his icicle ability instead (touch attack, 1d6+2 damage).
He's a fifth level cleric (+3 BAB) and has put all his feats into guns - Exotic Weapon Proficiency, Point Blank Shot, Precise Shot, Weapon Focus. +3 Dex, +2 Str. Last session we switched to the playtest gun rules. He has a musket, a pistol, and a blunderbuss so that he can get some kind of rate of fire going. Anyway, he got to fight a variety of bad guys (mainly from Carrion Hill - humans, summoned demons, Great Old Ones) with the new rules.
Our findings:
1. The touch attack was a nice addition, and it meant he could hit more. We felt that it was too distorting on highly armored opponents, though, and decided flat-footed AC at all ranges was probably a better approach.
2. Damage was still weak. Rate of fire is low so that x4 crit never came up. Other PCs that did not specialize in ranged combat at all way outclassed because of STR bonuses, especially at short ranges (which is the only place you get the touch attack benefit).
3. Reload time means you never reload in combat. And we're fine with that really; our group likes the "black powder feel" of the guns. It does provide a significant DPS differential though - if our cleric got quick draw then eventually when he gets iterative attacks he could put a couple in a round for like one or two rounds; meanwhile any goon with two feats and a bow can pop off two attacks at level one. He was the least effective combatant in the party as a result. He could do
round 1: 1 touch attack at 1d12 (musket)
round 2: 1 touch attack at 1d12 (blunderbuss, we treated as another musket)
round 3: 1 touch attack at 1d8 (pistol)
and then that's it. Find something else to do or reload a couple rounds.
Meanwhile another fifth level character could not spend the feat on the exotic prof and put it into Rapid Shot instead, and put less money than the guns take into a masterwork STR bow, and do:
round 1: 2 attacks at -1 for 1d8+2
round 2: 2 attacks at -1 for 1d8+2
round 3: 2 attacks at -1 for 1d8+2
and repeat forever.
Let's take an example where the touch attack really, really helps - say you're fighting ogres. AC 17, touch AC 8. That's as good as it gets, the guns have an effective +9 to hit. If your guy has a +6 ranged attack bonus, then with the gun he only misses the ogre on a 1. And let's also be super generous and say that the combat only goes three rounds so he's firing every round.
dps of the gun guy - missing only on a 1, 1d12/1d12/1d8 - is 5.17. dps of the bow guy, needing a 12 to hit (-2 rapid shot, +1 masterwork) and a +2 STR bonus, does 5.2 dps. So even with the most favorable gun setup, it underperforms the bow, and if you ever have reload rounds then it drops way, way, way under. And the problem gets much worse with iterative attacks, multishot, etc.
4. The guns are too pricey. The PCs already were eager to kill some gendarmes and sell their muskets; this doubled that incentive. You'd have to only put them on like level 6+ NPCs to make the wealth not totally disproportional. And you're pricing guns out of low level characters, which means they're even less likely to put feats into them.
In the end, everyone was pretty "meh" about the new guns. You can't put STR or other common damage boosts on them, you get a very low attack rate, and they do as much damage as any old sword. Or in his case, less damage output than his stock icicle power (by .33 dps).
I was reading the new Pathfinder Guide to Freeport and noticed the gun rules there do 3d6 (pistol) and 4d6 (musket), x3 crit. And at first that seemed high to me but once we looked at it, it's really not... As I actually use guns in my game and this isn't just an academic exercise, I plan to move the touch attack to flat-footed and boost to 2d6 (pistol) 3d6 (musket) immediately and then reevaluate. Doing the math in the ogre example, the guns then get up to 9.3 dps, which is still much lower than any of the actual fighty types are doing at level 5 but is at least something.
Now sure the gunfighter will push guns past this, but the various archer p-classes push bows way past this too. And if it takes an exotic prof (and is expensive and has misfire) then it should at least be somehow useful to Joe Character who decides to invest heavily in them but not be superspecialized or take a special class for it.