To think if you've planned to meet someone and you let them down ...

[RainbowsAndCrystals]

Then the least you can do is say sorry?

I would. Even if it wasn't my fault, for example being stuck in traffic. I'd still say - I'm so sorry I didn't make it, x y and z happened.

Yeah, I do like an apology and if it's me that's let someone down I apologise profusely because the guilt gets to me otherwise and I always do so asap.