The waterfall is amazing and completely worth the fairly frightening (to me) descent on slippery stone and wooden steps, completely worth the heart-stopping climb back up.
I came to Costa Rica during rainy season, expecting mostly clear days with one- to two-hour tropical storms. Unfortunately, I got caught in some freakish weather zone of almost-solid rain for a week. So my two days in the Arenal area did not include seeing the volcano or a trip to the hot springs, but I loved the waterfall.
I was on a pretty tight budget, so I decided to walk to the falls. Well. . . I don't recommend doing this. Of course, it was raining for about 3km; then for the remaining UPHILL 3km when it stopped raining, it was incredibly hot and humid. But even if it were a nice day, the scenery is nothing too special compared to what you'll find at the waterfall and it tired me out before even tackling the several hundred steps down to the falls. It was so frustrating to watch people passing me in cabs and on horses! It might be a bit better if you have someone to walk with. I found a dog that accompanied me most of the way. . . that was somewhat entertaining.
The falls costs $6 for foreigners, $3 for citizens to enter. This may be a price change because all the books say $3. At $6 you really want to make the most of it, so again, don't tire yourself out before getting there! You can hike further along the stream to where the current slows and swim.
I sat and stared at the waterfall for almost an hour, soaking my feet in the current. It was so beautiful, relaxing, refreshing, and gratifying. It was only my second day in the country and the overall vibe was. . . ahhh. THIS is what I came to Costa Rica for!
On the way back a kind British couple gave me a ride in their cab--it was $10 for them so I did feel a bit justified in trying to save money. 10 minutes after I got in the car. . . it started raining again!