To stay on topic, Writer Beware does not recommend any agent who charges fees, no matter how pleasant they may have been to work with. Agents should make their living by selling their clients work and collecting commissions. If the agent is collecting fees up front, it doesn't matter to them whether the book sells or not . . . they've made their money. Agents shouldn't be selling editorial services, own their own publishing company nor should they submit their client's work somewhere where they're getting a kick-back. All of these things are detrimental to the author who's counting on the agent to look out after their best interest.
Actually, we don't recommend agents either, but that's because every author's needs are different. We do recommend focusing on agents who've actually sold books in the area you're interested in writing in. If the agent isn't selling books to advance paying publishers, it doesn't matter if they're charging fees or not. An amateur agent is just as dangerous to a manuscript as a scamming agent.
It is not in any writer's best interest to pay up-front fees to an agent.
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Now, with regard to Slushkiller and formal rejection notes:
Slushkiller describes the Hayden's experiences with dealing with slush. It's echoed in Miss Snark's experiences, Colleen Lindsey's experiences, Janet Reid's experiences and many other agents who blog about dealing with slush.
Given the volume of queries, partials and fulls that agents receive from non-clients, there simply isn't enough time in the day for them to go through every submission and give detailed personalized critiques. Period. They deal with these after they finish working on their current clientele's projects - usually at home after hours. The only way they can go through the hundreds of queries they receive is to identify those they want to read through later and those that are not what they're looking for. The ones that get rejected way outnumber those they want to review later. The most efficient way for an agent to deal with something they don't want to represent is a form rejection letter.
This also helps avoid getting into arguments with the rejectee. Many agent blogs mention that personalized rejections seem to encourage the rejected author to write back, imploring the agent to re-read the query/partial/full, counting on the fact that if the agent only "spent more time with it, they'd see the (fill in the blank) that they missed the first time. When the agent replies that no means no, then the writer gets combative, which, of course, solves nothing and encourages the agent to forgo personalized rejections and copy a form rejection from a fellow agent from then on.
The pictures in Slushkiller show the volume of mail that the Hayden's had to deal with at Tor. It's sometimes quite instructive to actually see what an agent or editor may receive in a very short period of time.
So, while you may not find using Slushkiller useful, it is hardly "idiotic" to suggest that authors, especially new authors, review the information in Slushkiller to help understand why an agent might use a formal rejection letter.
Of course, your mileage may vary.