I feel really lucky to have DH's DCs in my life. DSS is 18 and DSDs (twins) are 12. They are all fantastic.
One of my DSDs has just stayed for a week and I actually feel really sad to be dropping her home later :( I'll really miss her! It's been lovely having her around and she's great with her little brother (11m) and sister (3.1) (well technically they're half-siblings but we don't use the word half), they all adore each other.
I'm so lucky. There were a few teething problems (DH and I have been together 8 years) with his ex but they accepted me pretty quickly. I spend lots of time with them even when DH is at work - I would never try to be a second mum, but I guess as I'm a lot younger than their parents I'm more 'fun' and approachable.
The girls were my bridesmaids in 2008 and DSS was one of our witnesses. I was too nervous to make a proper speech after but I did get up and say how grateful I was to them for welcoming me into their family.
Sorry for the self-indulgent post but I just wanted to write this down, I see so many posts about really tense step-families with lots of (understandable) problems and it's made me appreciate how lucky I am. :)