Well I split around 2 1/2 grams of 15x kratom extract with a friend of mine. He described it as "exactly like opium." I've never done opium and i was kinda stoned but it is really relaxing, like REALLY relaxing and everything gets quieter and you are just sooooo chilled out. its good stuff.
Erowid has experiences ranging from slight giddiness to blissfulness... A friend says its like a cold sweat, then bliss, then cold sweat, then bliss. Maybe at higher levels its really really mellow like Shock says?