At pollster.com, Thomas Holbrook has a useful piece summarizing current polling averages for 29 of the 31 gubernatorial contests being held this year. According to his calculations, Republicans are favored to pick up no fewer than ten Democratic governorships, but Democratic are favored to flip five Republican governorships, giving the GOP a sizable but not extraordinary net gain of five.
Looking more closely at the data, it's obvious that some of the turnover involves relatively predictable switches where open gubernatorial slots in strongly red and blue states are returning to type (a trend that Nate Silver noted some time ago). Oklahoma and Wyoming fit this pattern among red states (so, too, do Kansas and Tennessee, which were not included in Holbrook's analysis since there has been no recent polling in those states). Among blue states, Rhode Island, Connecticut, California and Hawaii have open Republican-controlled governorships.
Another pattern which is hurting Democrats is the backlash against the party in power at the state level in marginal territory. That helps explain current GOP advantages in PA, MI, IA, and OH.
The good news for Democrats other than their ability to offset large GOP gains with some pickoffs of their own is that the closest races could break in their direction. Holbrook has Republicans in the lead in FL, IL and OH; Democrats have made recent gains in the latter two states, and Republican candidate Rick Scott in FL has extraordinarily high negatives that probably set a low ceiling on his support. WI, GA, SC, and TX are definitely within reach for Democrats as well, with unusually strong non-incumbent candidates running in all four states (in GA, the serial ethics and financial problems of Republican Nathan Deal are a real wild card). A third-party candidate could make blue state Maine a sleeper for Democrats as well.
In any event, it should be remembered that gubernatorial contests are more likely to reflect local conditions than congressional races. I'm reminded of 1998, a relatively bad year for Democratic congressional candidates in the South, when nonetheless Democrats won upset wins in gubernatorial contests in AL, GA and SC. The "wrong track vote" in a midterm election isn't always about national politics.