Yes we did use WHFC. Love them.
We travelled with our (then) 4 and 6 yo's and my parents. We stayed at the Hilton which had a pool and a playground, as does the much more expensive (and much nicer) Sheraton... both offer adoption discounts. The other family who travelled with a child stayed at the WHFC guest house and ended up moving to the Hilton because their other son was bored at the guesthouse and they felt like they didn't have much privacy. Others LOVED staying at the guesthouse, so it all depends. For us the guesthouse is very nice but the nannies can be overbearing. It's one thing with your new child... They are so good with the kids and the attachment to them is obvious and can be a HUGE help there particularly with the older children and language issues. But your other child may feel pretty restricted in the guesthouse and the nanies will mother them too, which they may or may not welcome (mine enjoyed it at first but were over it quickly, especially dd because the nannies were always wanting to braid her hair and she hates having her hair done).
Food is easy to find if you don't stay at the guesthouse. But we spent WAY to much on bad food eating at Hilton (they serve bad, expensive American style food). We should have made use of Yonas or Mulat from WHFC more and had them take us to restaurants they recommended. Whenever they did we ate well for under $10 (for our entire party and treating the driver).Sometimes we got Ethiopian food (which we all love) but American food and Italian food are also prevalent.
It does get chilly. We were there in the summer/rainy season. And wore layers. Pants, t's and sweaters/hoodies worked great for the changing temps. I've heard the same from friends who travelled in the winter. When you go on your birthfamily visit dress in extra layers and plan on sleeping in your clothes and maybe not catching a shower at that hotel. Ours was fine but maybe not the cleanest I've ever stayed in.
We arrived in Addis around noon from our flight. Hilton picked us up at the airport. We called Mulat/Horizon House as soon as we arrived at the Hilton and he met us there 15 minutes later and took us to Horizon House. They had woken our son (then 2 1/2) up from his nap and as soon as we walked in the door (probably 1:30pm) they placed him in my arms. So once we were in Ethiopia it didn't take long to meet him at all. Because of the rains we couldn't do the birthfamily vist until our 3rd day so he stayed at the orphanage until we finished that and then came home with us for the rest of our time.
Hope that helps. I remember I had tons of questions. So feel free to ask away.