Some forms of instant replay have been a part of MLB now for a few years and I think that's how long you need to see if it's a good thing or not. I have mixed feelings on it still. But there is a place for it.
I am not a fan of a ton of replay reviews in any sport. The NFL is the worst with it. It has ruined the game in general as the biggest sport now has by far the worst officiating there is. The replays can be ruled "inconclusive" with too many ways out with too much time spent AND too much job protecting going on. And the whole "not reviewable" thing is just dumb. The NHL does the right thing I feel. They use it in scoring and rules violations. Perfect! Did the goal go in? Was the guy off sides? Things happen fast in the NHL and it's a great tool to get scoring right. It can also be effective in matters of the clock in other sports.
In MLB, they sort of have the right idea. I love it in cases of home runs, and fair or foul home runs. Geographically it's a long way away for the human eye, and it's a scoring play that happens fast. I don't like it on the bases or in a challenge situation, baseball games are long enough. I do like them in cases of rules violations. Possible runner, fielder or fan interference. I also think on the occasion of a scoring play at the plate they should look at them. Again, it happens fast, and it is a scoring play. And that's it!
Scoring and rules problems GREAT! This whole challenge thing is overkill. We have got to accept that the game is human and let it be so. If they get it wrong sometimes, that's the breaks. Play on. Too much replay slows every sport down and changes the flow of the game.
Use it wisely.