We are travelling from Addis to Bahir Dar, staying two nights, and then travelling on to Gondar next week. Skybus and Selam bus seem to use comfortable buses for these routes, but there is no online booking with either company. I have tried e-mailing them, but have got no reply. All I want to know is how easy it would be to get tickets to these destinations with just a day's notice, and similarly from Bahir Dar to Gondar. Also, which company is better, and what prices should we expect?

Can confirm. Just did the same. I rode both bus companies in the last week, and from my experience there was basically no difference between them.

Regarding your aim to reach Gonder from Bahar Dar, it will be impossible with these two companies because they don't stop on the way (except for lunch and toilets stops). These two companies are running luxury buses between some main cities and the capital city. You can't use them on the route except if you want to leave it.
As Gonder is easily reached by a good tarmac road, you can use a local bus of even a Hiace (Toyota style minibus). It's not a long way.
Michel

The only difference between the two companies is the price of the ticket.
As Selam is cheaper, Selam tends to be full before Sky but the price is not a problem at all for a tourist/foreigner.
It's different as you are Ethiopian...
Michel
Thanks Michel and leglock, much appreciated! Also, are there toilets on these buses and do we have to buy the tickets at the company offices or are they for sale in hotels or travel agencies?