I've been reading lots of comments from paying customers at the feedback page -- every one of them furious about the changes. I'd say about 90% are using Yahoo mail for business, and this is hurting them badly.
Fortunately, my business usage is fairly simple. Just a business address and a gif signature. I'm not having any major issues at the moment (cross fingers).
Some of the business users there are claiming significant disruptions to their normal activities, which is costing them money. Yahoo had better hurry up and get their act together with this before they face a class action lawsuit.
We have very high speed FIOS, but Yahoo mail is Veeeerrrrryyyyy SLOW, very slow on composition of new emails, screws up opening of emails, and gives annoying ads to boot.
Going back to Outlook. Pure email.