I can answer this from personal experience, so of course it may not always work this way....
Complimentary trines indicate that you get along exceptionally well. It requires no effort for two triney people to spend time together. They don't have to 'give' and 'take' much in the relationship - it all flows along without them even noticing. They could probably spend year apart and bond again instantly as soon as they saw eachother. They probably have a lot to say/do together, and generally find it comforting to be with one another.
Note that trines do not necessarily mean attraction. If you are talking about a 'friends' relationship, this probably works very smoothly, although they may not challenge, 'stretch' or expand eachother's realities too much. If you're talking about potential lovers, well.....there may just be no 'razz'
I have a very good friend - he was my twin brother's best friend - and his planets trine many of our natal planets. He liked me when we were younger, but I just couldn't concieve of him as a lover. Sorry - he got relegated to 'brother' material (he's now okay with this). We still get along exceptionally well, even though we don't see eachother often. I love talking to him, I want him in my life when we're old, wrinkley and cranky
but we will never be together
My opinion - you just need some squares and oppositions in a partner to make it interesting!!