What is the differences in rods with higher IM ratings than their lower rated counterparts? My understanding is that the higher IM rating, the more sensitive and light weight the rod will be, but the more brittle it will be.

Is this right?

What is the best type for different situations?

the higher the IM the more dense the blank material- improved sensitivity.  Not sure about being more light weight tho as the IM increases?

Wow the SEARCH feature here sure is accurate.....

Here try some reading from here in the forum....

http://www.bassresource.com/bass_fishing_forums/YaBB.pl?num=1194371160/4#4

Good Luck & Tight Lines!!!!  

Lightness is not given by the higher IM rating it 's given by the thickness of the wall and the ammount of resins used to bond the graphite fibers together.

Sensitivity is a highly subjective term, theoretically speaking, the denser the material the better it will transmit vibration, I say theoretically cuz in reality the perception of vibration transmission depends exclusively on your ability or the lack of it to detect it. Just as there are people that can stand more pain there are people gifted with a greater perception of tactile experiences. I swear I can 't tell THAT much of a difference between for ex an IMX and a GLX or the alleged superior sensitivity of the GLX is something I can 't detect.