I prefer looking at OBP and SLG separately. OPS combines two stats with different denominators that are on different scales.
From River Ave Blues:
The maximum OBP is 1.000, while the maximum SLG is 4.000. The answer, then, is to weight the statistics when combining. Forman went with (1.2*OPB) + SLG, and then placed that figure on a scale where 100 was league average. That made the stat easier to understand. Instead of having just a number, OPS+ put the number in context by comparing it to everyone else in the league. Now we know that when a player has a 120 OPS+ that he’s well above league average. We might not have been able to discern that by just seeing a, for example, .870 OPS by itself.
Improved as it may be, OPS+ is not perfect. For instance, Tom Tango believes that OPS+ still undervalues OBP, and that the calculation should be (1.8*OBP) + SLG. Even so, OPS+ is an improvement over straight OPS.