Last Novemer I built up a pair of 27+ wheels for my 29er frame. For rims I chose the very wide (45mm inner width) WTB Scrapers, and for tires I went with the WTB Trail Boss 3.0. I built them up on some used hubs I had lying around and went out to hit the trail. After noticing some chain stay rub, I had to switch to a 2.8 in the rear. The 3.0 clears my fork by only about 2mm on either side but I haven't had any problems there. Nearly 9-months later, I've ridden this setup in a lot of different conditions and have the following impressions to share:
Traction, traction, traction! Plus tires are grippy, due to the wider contact patch. This makes for improved cornering and climbing traction, which leads to more confidence in sketchy conditions. In loose-over-hard to loose dry conditions these tires railed corners and adjusted quickly to rider input. In some cases where the tires did break loose in a corner, it felt more like a controlled drift than a near crash. On hard-packed or damp, grippy dirt these tires roll well too, though there is probably negligible traction benefit when the dirt is near perfect. The one situation where the plus size is a drawback is mud. The tires have enough float so that they skim the surface rather than sink in to find solid ground. As I try to avoid riding muddy trails, I don't anticipate many issues with this.
Compared to the 29x2.3 WTB Vigilante tires I was running, I feel faster and more nimble on rough and twisty trail but slower on pavement and hardpack. The inflated diameter of the 27+ tires are only about a half inch less than the 29s, but that has a noticeable difference. My belief is that the diameter is so close, that the increase in absorption of the 27+ tire allows it to hold more momentum when encountering obstacles, while having just enough extra drag and smaller diameter to feel slower where there are no obstacles. I'm looking forward to building a frame that will take 29+ tires so I can compare.
Bottom line: I am having a ton of fun riding 27+, and isn't fun the ultimate unit of measure? If you want more smiles per mile, try plus.