We have 'one of each' - DS is 2.1 and DD is 10 months. Whilst it's 'ideal' for us, and I feel very lucky as we always wanted the chance to parent a boy AND a girl, it doesn't mean it's ideal for everyone. Nor does it mean it's ideal for the DCs.
DS adores his baby sister, and because of the small age gap I really do hope they grow up close, but a lot will be to do with personality etc. I often wonder if they will wish when they are older that they had a brother/sister to hang out with instead. I quite envy friends who have 2/3 boys or 2/3 girls, it always seems like they're in to all the same things and having a blast!
I actually like the fact though that we have/will have a wide spectrum of toys available to each DC, both traditionally boys AND girls toys, although as someone pointed out above it does make it quite expensive, ditto clothes (although DD does have a few of DS' hand-me-downs!).
I often get stopped by people (particularly old people!) when the DCs are in the double buggy and they coo and say things like "how LOVELY, a boy and a girl, you did well there!", to which I am always a bit bemused, as I just think "so would DD not be so lovely if she were a boy?" 
In MY ideal world though, i'd have 4 DCs- 2 boys then 2 girls so have best of all worlds. Never gonna happen though! 