I don't think that an e-mail offer is that strange. I've heard of offers through both e-mail and phone calls from legit agents who got their authors good deals later. It's just a matter of style. Now if they don't want to talk to you on the phone afterwards, that would be weird.
My rule is that if you feel at all uncomfortable with the agent, don't go with them. The beginning is when you're going to feel most enthusiastic about your agent, so if you start with doubts in mind, I would say no. Doubts could come from BR&BC, from their lack of enthusiasm in other projects of yours, what exactly they say they love about your project, the revisions they plan for your novel not sounding right to you, a lack of sales in your genre, etc.
When I first was querying, I queried any agent who took my genre and wasn't
bad, according to BR&BC. And the agent who offered me rep wasn't
bad, in fact he was a superstar in some markets. But he didn't have sales in my market or genre, his enthusiasm for my novel seemed limited to what was presented in the query letter, he showed no interest in what else I was working on, and he was incredibly vague anytime I asked what sort of revisions he saw for the novel. I signed with him anyway. I quickly regretted it.
Now when I query, I look for more than whether the agent is
bad. And I know that after this experience, there are agents who don't have a bad reputation, but who I would never say "yes" to if they offered representation. I don't query those agents.
The best, and possibly the most difficult, way of finding out about how an agent is is to communicate with a writer whom they currently represent. You will find out things you never will find out anywhere else. People are pretty willing to offer up info if you ask.
G'luck.
Just a note that this is not a foolproof plan. I've been burned by an enthusiastic author who was later unhappy with the agent. A longer history between the agent and author is probably key.
