We went out with the crew of Lava Kai in February and it was an awesome experience. It was a wet early morning due to the light rain we were experiencing. But then, morning rain in this area is not uncommon. The seas were a bit rough and we had no major issues getting out of the boat ramp onto the open water. We were fortunate that there were two flows going, one that had just recently started and that's where we headed first.
The boat captain (Shane?) is well experienced, having been coming out to view the lava by boat with his dad as a child. When we reached the newer flow, he kept the boat moving, making passes close to the flow. We could feel the heat of the lava and it was awe inspiring to literally watch the island grow. We've been to the official viewing point on land for the ocean entry, and it is great as well, but can't compare to seeing the flow this close. The land viewing can be hit or miss, each time you go being unique based on how active the flow is and what masses have accumulated or fallen in to the ocean when you happen to be there. Seeing it by boat, virtually ensures a great view!
While the volcano has been constantly erupting since 1983 (see http://hvo.wr.usgs.gov/kilauea/summary/main.html for the history) we don't always have an ocean entry so if you are visiting the island, this is one of those "don't miss the opportunity" moments.
Best part of the adventure is seeing the ocean entry so close. The crew was great, and very informative.
Worst part, it can be a very bumpy ride and you will quite likely come home SOAKED. So don't plan on doing this and then heading out for the rest of your day. Find somewhere to take a shower and get dried off, unless of course your next stop is the beach for swimming! All the warnings associated with roller coaster rides (and probably a few more) apply here.
I'd also be remiss if I didn't say that this ride isn't without risk. The flow can result in explosions that throw molten lava flying. We decided that the low probability of something really bad happening was worth the risk.