Honestly I think similarity is a bit of a red herring. The most interesting question is whether there are other beings we can communicate with in any sense of the word, so that we may learn from each other. For a somewhat trivial example, if we found robots on another planet, we would not consider them "life", but it would nevertheless be an incredibly important discovery.

On the other hand, it's of course imaginable that there are beings that we would in principle consider intelligent agents, but who exist in a way that in practice we have no hope of recognizing as such. Again to pick a somewhat trivial example, if galaxies were in fact intelligent beings that take billions of years to form a single thought, we may both in principle be very interested in communicating with each other, but in practice could never even hope to recognize each other as sentient beings, because of the intense difference in time scale.