My experience across a range of routes is that recent tightening of E+ access rules means that there is almost always E+ availability for those entitled to it. Unless you are flying primarily elite-heavy routes (e.g., SFO-ORD on Monday mornings), you should reasonably expect buying E+ Access will get you an E+ seat on really almost every flight you fly. If you're more than a few days out when buying your ticket, there will be some aisle and many window seats to choose. Of course, there's no guarantee, but IME, the availability is there.

I just purchased E+ Access for Mrs. PVDProf (who needs access herself when we fly with Master PVDProf), and I expect she will almost always be able to use it; I wouldn't have spent the money if I thought she'd still be in E- half, or even 80%, of the time.