This was the only tour operator on Vis Island that I found that would do a day kayaking tour, so not much choice. Nonetheless, they were very helpful in communicating with me before the tour, especially as I was travelling alone and let me know when there were enough people so I could join the group (6 others so I shared a kayak with our guide).
The kayak trip itself was excellent, and we started early-ish in the morning and had a gentle paddle to Stiniva Bay, which was relatively quiet when we arrived mid morning so that we had the chance to have a good swim before the boatloads of tourists arrived. Lunch was a simple tuna pasta we brought along and then it was paddling back near where we started (Rukavac) and onto the Green Cave.
Eventhough fairly busy, I thought the cave was pretty magical with this beam of intense sunlight coming right through a hole in the cave creating a glittery, effervescent effect in the water. We got out for a swim, which was fun darting around and through the column of light.
As we were going pretty quick, we had time to go round the island of the Green Cave and en route back to Rukavac and then our transfer back to Komiza.
Our guide (I think he was self employed by the company) was excellent (even though he kept saying his English wasn't good, it was pretty fine) and we had good chat along the way and he led the group confidently. From what I could tell, everyone in the group had a really good time and we finished about 2 pm or so. I was staying in Vis town (rather than Komiza) so they dropped me and a couple of others somewhere where I could walk 5 minutes into town.
I'd highly recommend the kayaking tour (375 kunas I think) as a great way to see these places and see Vis from a slightly different perspective (along with getting a scooter to scoot round the island).
I also went on quick tour (1 1/2 hours) of the Blue Cave with Alternatura and it was fine, although not particularly inspiring. Fun speed boat ride (20 mins or so) out to the island where you hop onto a small boat to go into the Blue Cave. Perhaps because the day was overcast and we were there earlier (10 am) the cave was blue but not spectacularly so. Also, not much commentary or anything provided so you're just really paying for the ride there.
p.s. Once you've taken a tour with the company you automatically get 15% discount on subsequent tours so it would be a good idea to maybe do a cheap tour first before doing any more expensive ones like kayaking.