The first part of the hike is easy and brings you up the Pacific Creek basin to a grassy area in the 12,200 region. This is where Atlantic's west ridge route starts. That ridge looks easy and tempting, but stay strong. Fun climbing is not far away.
Just beyond the intersection with Atlantic, the grass turns to rock and you find yourself at the base of Pacific's west ridge. Woo hoo! Climbing! The route up the ridge is not entirely obvious from the bottom; however, you get a pretty good idea of where you ought to be going.
One caution: the rock is not as solid as it could be. Football-sized rocks rest on tiny rocks, and when you step on them, the whole group begins to slide down the hill. If you climb this route and someone is climbing above you, keep your eyes and ears open.
Parts of the ridge look very hard. If you stay at the ridge crest you run into harder Class 3 climbing, but its often possible to scoot around the harder stuff by hiking up loose dirt gullies. If you stay on the ridge crest you get some exciting exposure, especially to the north. When you get near the top there is a nice climbing section that brings you up to a notch between main summit and false (west) summit. Climb down into the notch and be sure to peek down the north face- waaaaaaay down! Then make the easy final climb up the main summit to the tippy-top.
I summited Pacific at about 9:30 after a 3-hour climb. The ridge itself took about 1.5 hours for a 0.6-mile route.
After that, the rest of the route is cake- Class 2 stuff. Atlantic is very gentle, and the climb breaks naturally into about five 'pitches'- sloping parts with small flat parts in between. Atlantic has two summits, and the west one is where the summit register is. I think it's slightly higher than the east summit. In any case, the two summits are 30 seconds away from each other across flat terrain, so don't feel compelled to visit just one.
It takes about 45 minutes to get from Pacific Peak to Atlantic Peak.
Atlantic's west ridge makes for a nice downclimb. I tagged along with a nice group of hikers from the Colorado Mountain Club- they recommended stying near the ridge crest for as long as possible. I'd agree with that method. The side of the ridge is plain old talus that would probably be a pain to contour on. Besides, the ridge is pretty, and you get great views of Fletcher. There's also an old mine building stuck in the middle of Atlantic's west face, looking like a single dart stuck in a dartboard.
Atlantic's ridge does have some tedious talus downclimbing for the last bit before the grassy area at 12,200 (or so). But it's a small price to pay for an otherwise easy downclimb. Once at the grassy area, it's a lovely stroll back down the hill to the gulch (and the willows). There's actually a hiker's trail that runs along Pacific Creek; it's easier to spot coming down than it is climbing up.
It takes about two hours to get down from Atlantic. Including the hike down the Mayflower Gulch Road (1 mile), it took me 2:10 to go from Atlantic to trailhead.
All in all this is a sweet, if slightly loose route. It is a fitting route to such a shapely peak. Just watch out for rocks and start early!