OVR is a good indicator to start with (I always look at it first). But it can trick you. Once you look at the OVR, you need to look at the individual ratings relative to the position you need him to fill. Do those ratings help you? Keep in mind that OVR is going to indicate the amount of money he is going to want to be paid, either now or later.
Everybody knows that HBD ratings are king, and they all try to get the most highly rated players, by whatever means (draft, IFA, trades, FA). So you pretty much know every other owner is trying to fill his roster with as many highly rated guys as they can. So how do you win consistently?
By then looking deeper. I have received very good value back on guys with OVR that are low. A relief pitcher with an OVR of 58, but has the right core ratings to give you a goose egg inning every other game. A backup infielder with an OVR of 61, but the can hit lefty's and run the bases well with speed, or pick it.
If you look at teams rosters, you will see the great teams with about 5+ guys over OVR over 80. Then the thick middle of their roster will be populated with about 10-15 guys in the OVR 70-80 range. The superstars and the core. But that isn't what makes the team great. It is the bottom guys, the ones with OVR's of below 70 (usually about 5 of them on any given good roster). These guys are handpicked for a reason. They are cheap, and they have core ratings that help the team win, that make the team better than the other owner who got his 5 80+'ers, along with the ML average core, but neglected to look deeper.