It is extremely easy to get a visa upon arrival. The visa queue is just before the immigration checkpoint, on the left. Exit the plane quickly to get to the front of the line. You need two passport-sized photographs and $20.
Get Birr at the Dashen bank just before the immigration checkpoint, on the right. $250 a person should do it, unless you buy souvenirs. Keep your receipt in case you need to exchange your Birr back again. Also, there is a bank at the Ghion Hotel, if you are staying there, and you should exchange your Birr back to your own currency in Addis, as the airport bank will be closed when you leave.
There are many taxies outside of the airport, on the left. I took a blue and white taxi and negotiated a price of 60 Birr, or about $3.60. Every driver I used spoke English and was helpful. In fact, most of the people I met spoke English.
I went in in mid-October, just after the rainy season ended, so weather was never a problem. You might want to check reviews by people who went during the rainy season, since so much time is spent outdoors.
I suggest going when there is a religious holiday, particularly Timket, which falls on January 19.
Remember that this is a third-world country. You will not be staying at four star hotels, most of the roads are unpaved, the water is not safe to drink (always use bottled water, including on the plane leaving Addis), and Ethiopian food is not to everyone's taste. We paid for our own lunches, and almost everyone chose overcooked pasta rather than eat Ethiopian food. Breakfast was usually fried eggs, toast, and bread. Vegetarians will be pleased, as Ethiopia has many vegetarian dishes. Contrary to what the guide books say, beer is inexpensive (no more than $1, and usually less). I particularly liked St. George.
The hotel descriptions are accurate. The one in Debark is described as basic, which meant no screen in the window, no toilet paper, no towel, no stopper in the sink, no hot water (most of the hotels relied upon solar heat, which was a problem on an overcast day), intermittent cold water, and intermittent electricity. However, the bed was good and the door locked. What else do you really need? Bring a microfiber towel (you'll use it after crossing the Blue Nile), toilet paper, and, if you do your own laundry, a sink stopper and detergent. However, every hotel outside of Addis charges pennies to wash clothing, in some cases doing it overnight. Since the trip is extremely dusty, pack enough clothing so that you'll have something to wear if the hotel takes two days to do your wash.
The risk of skin damage is high because of the attitude. The UV index was around 13, so bring a wide-brimmed hat and wear good sunscreen. During the dry season, when I went, it felt warmer than the official temperature, so bring short-sleeved shirts.
I took malaria pills and brought insect repellent and a mosquito net. If I were to do it again, I wouldn't bother. I never saw or heard a mosquito because almost every place we traveled was above 2000 meters, where mosquitos don't fly. On the other hand, many of us were bit by fleas, so bring Benadryl lotion.
This is a somewhat more challenging trip than described by the trip notes, but only because of the hikes and one particularly poor hotel in Debark. I brought a pair of trekking poles with me, which were useful when walking to Tississat Falls and even more useful on the hikes. The one in the Simien Mountains should not be tried by anyone with a fear of heights. The one to the Ashetan Maryam monastery should not be tried by anyone with a bad back (because of the mule ride). You can walk up, and two of our group did it, but you should be in reasonable shape.
Since I came from the U.S., I didn't take the charter flight. I flew Emirates to Dubai, stayed there overnight, and then flew to Addis. This took a little longer than Ethiopian Air, but I was fearful that I would miss the trip if there was a problem with Ethiopian because they had only one flight a day. Also, Emirates gave me two inches more of seat room, slightly better food, and a superior entertainment system. Others who took Ethiopian said that it was first rate; certainly the internal flights were good.
Ethiopians are friendly (especially in the north) and the ones we met were honest, although two of our group had their pockets picked in Addis. In the first instance, an Ethiopian woman grabbed the thief's wrist and made him return the wallet.
People love having their pictures taken (although priests in Lalibela will ask for a few Birr, and certainly you should ask before taking a photo). If someone approaches you to have a picture taken, don't offend them by offering them money. What they really want is to see themselves in your viewfinder.
Ethiopians will attach themselves to you and try to be useful in the hope of being given a tip. For example, you'll probably need help to cross the Blue Nile (part of the hike to Tississat Falls). Give a minimum tip of 10 Birr. You can cross while barefoot, but Tevas would be safer. If you let someone accompany you on the hike, assisting you where the trail is rough or carrying your daypack if you become tired, give a minimum of 100 Birr. I gave 200. You can cross while barefoot, but Tevas would be better.
Unfortunately, there is begging. Don't give children any money, because that only encourages them to skip school. Kids will ask for money, then for your shirt or hat (which they will sell), then for your empty water bottle (which they will sell for about half a penny), then for a pen. If you must, bring inexpensive ball point pens to hand out.
Finally, I suggest that Exodus skip the trip to the south; Wendo Genet is not worth the time. On the other hand, I wish that Hadar had been included.