How frustrated should I be that I couldn't place my book with an agent or publisher but now that I've published it as an e-book on Amazon it gets reviews like, "One of the best mystery books I've read"? Should I feel more or less frustrated than I feel already?
Don't feel frustrated.
Be glad there are ways to get your book in front of clearly discerning readers and away from the obviously taste-free agents and publishers of this world, who wake up in the morning with one goal in mind: thwarting your creative genius.
Ok, shark snarl aside, you're looking at apples and asking why the orange vendor didn't buy in bulk.
My job is not to find good books. My job is to find books I can place at a large publishing house for wads of cash. I pass on very good books every single day maybe even yours.
The market is a fickle beast, and publishers and editors go through phases of what they want to buy just like everyone else. There are lots of VERY good books that I can't sell. I usually pass on them with a reminder to query other agents. Maybe it's me. Maybe another agent can sell it. But when agents rally round the bar and exchange war stories, one common topic is what we're having a hard time selling.
If you're writing books people like to read, more power to you. Continue to work to develop your craft and build your audience. The best way to show us how wrong we were to say no is to spell it out in hundred dollar bills. Satisfying too.