E-mail Queries: Efficient or Effective?

​When querying an agent, I like the ease of the e-mail query. I can research agents, change my query letter to match the agent's taste and send out queries to a couple of agents a day. And I'm being environment-friendly: no paper for the query, no envelope for mailing, no SASE, no form rejection letter to hole punch and insert in my rejection binder. Very efficient but is it an effective way to submit? My guess is that with the popularity of e-mail queries, agents receive many more queries and are spending a lot more time reading queries and sending out rejections and not able to spend as much time selling manuscripts and working with authors. Look at the April stats for agent Elana Roth of Caren Johnson Literary or for Nathan Bransford of Curtis Brown. Does this frustrate agents? Is the percentage for rejection as high with snail mail queries? Are authors more thoughtful in their research when sending through the mail? I don't know. I do know I can receive replies sooner with e-mail than through the mail. Again efficient. I also don't know the end of my story yet with submitting A Kiss and a Curse. When it has been accepted and published, then I can tell you the story of how many agents I submitted to and how many editors and if those submissions were by e-mail, by mail, by searching on the internet or meeting the agent and editor in person. Until then I don't know if e-mail queries are effective.