A few thoughts: A story about diamond thieves, secret agents, and double-crosses meets a giant alien jellyfish monster that feeds on carbon (particularly found in diamonds and coal). Definitely sounds like it could have been a huge mess, but Ishiro Honda does a great job at combining these two stories and making them complement each other instead of just sharing the same space. The special effects in this one are a bit of a mixed bag. Some shots look great, but others are pretty weak. The creature itself generally looks pretty awesome, but the use of miniatures ranges from effective to cornball -- very true to form for a kaiju movie of the '60s. The story hits a few dry patches, but is mostly fun to sit through, as I found myself thinking this should be a lot more popular. I guess not putting Godzilla in every movie sometimes hurt Honda's lasting success, but I applaud how much he branched out and tried new things for the genre like this. It's a unique monster, and overall a pretty solid movie.
Who would I recommend it to? Fans of Ishiro Honda, Godzilla, Gamera, etc. It's not a guy in a rubber suit, but it fits in with those movies quite well.