Just thought I would throw my 2 cents in...
First of all, I have multiple Macs, and have tried out the GW2 Mac client on all of them. My brand new 27" iMac runs this game phenomenally! It is the baseline i5, 6970 1GB machine with an upgrade to 12GB RAM (4GB of which is pretty much constantly used by Parallels) runs it at about 50FPS in most circumstances, 35-40 in high density areas or heavy pvp.
My new 2.6Ghz 16GB RAM rMBP runs it fine as well, but only when put in 2880x1800 retina resolution with subsampling ON. I run it on high settings otherwise and get 60FPS pretty much at all times, dipping to 40FPS or so when in high density pvp or big fights.
Finally, I have a 15" mid-2008 MacBook Pro (just before unibody) with 4GB of RAM, 2.4Ghz Core 2 Duo and Nvidia 8600GT. This is well before what they consider minimum spec but I was absolutely SHOCKED to find out that it will run the game at 30FPS on all low settings (1440x900 resolution) in main areas. When in heavy php or high density areas it does drop to about 15-20 FPS but it is still surprisingly playable.
For a non-optomized cider wrap this client is incredible! I have never seen such an easy transition, and the emulation is pretty much spot on. So far I have only tried using boot camp with the iMac, and naturally, it blew away the mac program in terms of FPS. I got a constant 60FPS at all times no matter what I was doing, and it would have been higher no doubt if I were to turn off Vertical Sync, but there is honestly no reason for me to waste time transferring back and forth in boot camp when I can run it just fine under OSX.
The iMac and rMBP are running OSX 10.8.2 and the older MBP is running 10.7.5. It is very clear that Arenanet has high standards for their client to make sure that their minimum system specs mean that it can be run WELL in all circumstances, because my 5 year old MBP can run it quite decently, and it isn't included.
For those interested in a review of the game, I'll give a quick quip of it. I have played most MMO's since the original Everquest came out, including EQ, EQ2, WoW, Star Wars Galaxies, SWTOR, Rift, EVE, and many, many others. I will also say, that while it has some minor similarities to many of these games, it would be entirely undeserving of this game to compare it to any of them. It is a very fun game that ties together world/zone events (Rift-style), full style crafting, fun pvp, and a very unique style of combat. The combat system is WoW-esque, but differs depending on your type of weapon. Depending on your play style, you can kite, or bash your face into mobs, or do more of a CC type play. The weapon system would require a beast of an explanation so I won't go in depth, but suffice it to say it is very unique, and very cool.
Gameplay is very fun both solo, and in groups, and the quest line is intriguing while also not being completely overbearing. World exploration is a key factor in the game and is tons of fun. Crafting is pretty general, although a very cool thing about it is that you can start one profession, then drop it to pick up another, then go back to the first one without losing any of your recipes or levels. You essentially pick up where you left off. Also, there are no gathering professions, anyone and everyone can gather anything with the correct tools.
Overall I am very impressed with this client for being a Beta Cider wrap with no optimization, and the game is pretty fun as well. For anyone who is bored with their other games or just wants something new, you could certainly do a lot worse than GW2.
