Excellent combination of the most important landmarks in Iceland! The only thing i did not like so much was that we were in Gulfoss so little time..... Understandable because this is a tour... but if you want to have some more time in Gulfoss you should take the Golden Circlo Tour in the morning...not in the afternoon. Nevertheless, this one (Blue Lagoon, Gulfoss and Geysir Express) is worth going and taking it if you are in Reykjavik for a short period of time.
Most hop-on hop-off buses offer the mos important places in a city... Well... this is different... It is ok if you want to get a general idea of the sorroundings of Reykjavik but it will not show you many important places of downtown... Expensive for what it is...