My friends and I went to Hells Gate on our trip to Africa a few months ago, and overall it was worth the trip. Good hiking with impressive views. They offer 3 different hikes (a short, medium and long). We did the medium version, which took us a few hours and involved scrambling up and down some sharp slopes but nothing too extreme.
The main reason I'm writing this review, however, is to warn people about the bikes! We could have driven down to Hells Gate for the hike. But we elected instead to make a day out of it and rent the bikes at the national park entrance and bike to the canyon. It's true that the scenery is great and you even see wild animals like zebras, warthogs and antelope up close. But the bikes rented by the national park are crap!! Even after carefully picking out our bikes (some brakes didn't work, some tires really needed air), we still had problems on the way there. It's over 5 miles to get to the canyon from the entrance. . . and it's all downhill on the way to the canyon and all uphill on the way back. We only made it halfway down before one of the bikes blew a tire and we had no way to contact anybody to bring a new one.
There was hardly anybody driving on this road so it was half an hour before we saw anybody. We flagged down the first bus that came by and inside it were 2 other people in it who had also rented bikes and blew a tire! They had to bribe some guy to drive them back or they would have been stuck too! Luckily we caught a ride with them back to the park entrance and then paid our driver to actually drive us to the canyon so we still got to hike.
If you really want to bike to the canyon, then make sure to check the treads of your bike so that you know you're not getting tires that are already worn out. Even if you check the tires, the road is really rocky so be prepared to walk back if you have to.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.