Appalled at the bad ratings some visitors gave the park because they did not get to view the bison, I would like to provide some tips on how to increase your chances. In fact, the bison are there year-round, but - as some have correctly pointed out - they are given space; which is in everybody's interest, not least the bison's. While there is a conveniently located "bison overlook" right off I-70 (exit 254) to the north side of the freeway, seeing bison from there is not a given; which is just okay if you think about it. Hey, this is not a petting zoo where you expect your fair share of coat between your fingers for the dollars you spent. These are semi-wild creatures that roam a spacious pasture and are not meant to be displayed on cue. So how to go about viewing the iconic symbols of the West if they happen to be too elusive for an overlook experience?
First, check out the North Pasture (on the north side of I-70). From said overlook, a paved footpath/cycle lane continues along I-70 and runs the entire length of the pasture (roughly 1 mile). Following this path will give you plenty of opportunities to stop and scan the pasture - make sure you also take in the wooded areas. This is definitely not possible when whizzing buy on the interstate, so you will need to take a little walk. The path ends at the Chavez/Beaver Brook Trailhead (with Stapleton Road for access). There is another roadside overlook there (I-70 exit 253). A camera with some zoom might come in handy, a phone might not allow you to take good photos if the bison are farther away from the fence or among the trees.
Second, if you really fail to spot any bison in the North Pasture, try your luck in the South Pasture (south of the interstate). You may consider driving Genesee Mountain Road or driving to the Patrick House Trailhead. Both locations are easily accessible and border the southern bison enclosure. An option that might provide you with a more intimate experience with the land and the animals is hiking the Genesee Mountain Trail (Outer Loop) or the American Bison Trail, which both start from the aforementioned road/trailhead and run along the fence line separating the bison pasture. They can each be completed in 1-2 hours; alternatively, you can always hike (and stop) at your discretion.
My rating of the park is 4/5 because the downside I see is all the development in the area, which makes it difficult to take photos of the landscape and/or the bison without having fences, power lines, the interstate, private estates, etc. in the picture. You also hear the droning of the traffic on I-70 quite a ways off into the greenery. But while this obviously is not Yellowstone NP, it is still remarkable that there is such a splendid natural area within a 15-minute drive of one of the biggest metropolitan areas west of the Mississippi! We should embrace this!