I used to have a Garmin 496, and its obstacle alerts were always consistent and spot on. My new iFly 740b is, well...significantly less so, I ahve the obstacle alerts set to strict, yet several times it has failed to alert me to obstacles until WAY too close.
Two days ago, I was flying along at 2000ft and suddenly the iFly screamed at me. I looked down and it said "OBSTACLE 0.4NM, 10 SEC". I saw the tower just off the tip of my right wing, maybe 200ft below me and VERY close. WTF? The red circle around the obstacle on the map THEN turned on. If I had been 1/8 mile to the right and 200ft lower I would have hit the tower before the unit alerted me. That's not acceptable.
I circled back around, descended a few hundred feet, and pointed the airplane right at the same tower. The map circle was on now (it actually didn't turn off), but the iFly *never* gave me an audio or visual alert. I turned away from the tower (pretty damn close) before I ever got a warning.
I like the unit overall, but don't trust it now AT ALL to alert me to obstacles or terrain.