This is "field day" season at test plots conducted by seed companies, co-ops, vo-ag, vo-tech, university extension, etc., etc. These plots can be of value if several points are considered:

Field variability alone can easily cause apparent differences of 10 to 50 bushels per acre. Be extremely wary of test plots that are not replicated, or only have "check" or "tester" hybrids every 5 to 10 hybrids. The best test plots are replicated (all hybrids repeated at least twice, preferably three times).
Be careful with test plots where one company's hybrids dominate the entries. Odds are stacked in their favor!
Don't put much stock in results from one location and one year, even if the trial is well run and reliable. Use multi-environment averages whenever possible, because you never know what next year's environment (weather, soil conditions, planting date, etc.) will be like.
Don't over emphasize results from one type of trial. Use data and observations from university trials, local demonstration plots, and then your own on-farm trials to look for consistency of hybrid performance.
Use field days to make careful observations and ask questions, but reserve any decisions until you have "seen the numbers." Appearances can be deceiving. A few suggestions:
- Walk into plots and check populations. Hybrids with large ears or two ears per plant may have thin stands.
- Break ears in two to check relative kernel development of different hybrids. Hybrids that look the most healthy and green may be more immature than the other hybrids.
- Differences in standability will not show up until later in the season and/or until after a windstorm. Pinch or split the lower stalk to see whether the stalk pith is beginning to rot.
- Visual observations of ear-tip fill, ear length, number of kernel rows and kernel depth, etc. don't tell much about actual yield potential. Some "ugly" hybrids are good performers.
Buy hybrids ...don't be sold on fancy "result" books, plot signs, flags, streamers, caps, brats, etc. Take your seed salesman to task. Ask him to provide multi-environment averages and some measure of consistency for the hybrids you are interested in from his company, and if possible other competitor hybrids.