I can concur with the reviews on here. It is highly recommended. It might be a long day but well worth it. Not that I have a problem, but toilet/refreshment breaks are well planned/timed. With the first break at the supermarket/bakery nice and early and in daylight.
All the stops are interesting in their own way. My highlight being the Secret Lagoon, I never thought I would get in a thermal spring pool, but I'm so glad I did (they do provide swimwear and a bathing towel at a cost of course), I thought I'd freeze and that put me off, but no, I even walked around the outside of the pool and viewed the thermal springs in my swim shorts, Fantastic eperience.
Also the Geyser at Strokkur. With frequent erruptions. It is a popular spot with many tourists, but I had time to watch film the activity as well as going to the restaurant.
The timings are pretty good.
Asa (guide) and Loukas (driver) were great hosts and happy to impat their knowledge. Top job.
Considering I was a little bit hesitant about going, I need't have been.
Don't think twice about booking it, you won't regret it. Just make sure you have good outdoor footwear.