There are many reasons for distrusting the information the IPCC is giving us on climate. Serious contention has arisen from 2 specific areas: The IPCC exaggeration of glacier melt in the Himalayas and the association of climate change with disaster damage such as hurricanes. The IPCC affirmations on both these areas are being seriously contested.
Professor Roger Pielke Jr. is an example of the debate that is raging, and gives a serious critique of the IPCC and it’s flaws with regards to information acceptance and delivery. This is how he concludes:
“That the IPCC has made some important mistakes is very troubling, but perhaps understandable given the magnitude of the effort. Its reluctance to deal with obvious errors is an even greater problem reflecting poorly on an institution that has become too insular and politicized.”
He also has a very detailed piece on the conflict of interest of Rajendra Pachauri and why he cannot be trusted either. Read it here. These are one persons opinions, take them for what they are worth.