What's the correct level of difficult for the next "challenging" piece?

[Bob]

Just wondering in general.

My basic concern was this -- Do you think it's good to play a piece that you can't read well.  The kind of piece you must "decipher" note by note, chord by chord.  Something that takes a lot of brain power to take in.  Is that good to do?  You really have to just memorize those pieces, don't you?  You don't painstakingly put them together and then suddenly you start reading the piece, do you?  It's more like the piece becomes pounded into your fingers and you play while glancing at the music, right?  Is that good?

It's kind of a vague question.  The poll isn't greatly constructed.  I'm just looking for ideas.

[rimv2]

It's what ah did for Rachmaninovs prelude in C#m a while back. But it was just mah bad sightreading skills.

[Bob]

Hmmm... looks like people like the big challenging pieces that take months.

[rafant]

Well, I always must learn my pieces note by note. There is no other way known for me, and so all my pieces are challenging. But certainly I pick up my pieces among those ones able to take me to a little higher level. For example I'm learning currently a full left-hand arpeggiated piece, something I never had attempted before. Next one to learn is a full poly-rhythmic one, something new too.