In the IPCC's own words

The IPCC produce “The Physical Science Basis” that they know few read or understand. It’s completed and set aside while a Synthesis Report or Summary for Policymakers is written and released at an orchestrated press conference months before the Science Report. The two are markedly different.
The Summary claims that computer models are scientifically sound. Media focus on temperature projections invariably putting the highest increase in the headlines. These projections are predetermined, and always wrong. The Science Report explains why they cannot be correct. Here are 17 quotes from Chapter 8 of the 2007 Science Report.
Nevertheless, models still show significant errors. Although these are generally greater at smaller scales, important large-scale problems also remain.
Due to the limited resolutions of the models, many of these processes are not resolved adequately by the model grid and must therefore be parametrized. The differences between parameterizations are an important reason why climate model results differ.
Since the TAR, there have been few assessments of the capacity of climate models to simulate observed soil moisture. Despite the tremendous effort to collect and homogenize soil moisture measurements at global scales (Robock et al., 2000), discrepancies between large-scale estimates of observed soil moisture remain.
The global Aerosol Model Intercomparison project, AeroCom, has also been initiated in order to improve understanding of uncertainties of model estimates, and to reduce them (Kinne et al., 2003).
Our assessment is that although problems remain, climate models are improving in their simulation of extratropical cyclones.
Unfortunately, the total surface heat and water fluxes (see Supplementary Material, Figure S8.14) are not well observed.
These errors in oceanic heat uptake will also have a large impact on the reliability of the sea level rise projections.
Evaluation of the land surface component in coupled models is severely limited by the lack of suitable observations. Large discrepancies remain in albedo for forested areas under snowy conditions, due to difficulties in determining the extent of masking of snow by vegetation (Roesch, 2006).
The evaluation of the hydrological component of climate models has mainly been conducted uncoupled from AOGCMs (Bowling et al., 2003; Nijssen et al., 2003; Boone et al., 2004). This is due in part to the difficulties of evaluating runoff simulations across a range of climate models due to variations in rainfall, snowmelt and net radiation.
Despite considerable effort since the TAR, uncertainties remain in the representation of solar radiation in climate models (Potter and Cess, 2004). Several other groups have evaluated the impact of coupling specific models of carbon to climate models but clear results are difficult to obtain because of inevitable biases in both the terrestrial and atmospheric modules (e.g., Delire et al., 2003).
Blocking events are an important class of sectoral weather regimes (see Chapter 3), associated with local reversals of the mid-latitude westerlies. There is also evidence of connections between North and South Pacific blocking and ENSO variability…but these connections have not been systematically explored in AOGCMs.
Despite this progress, serious systematic errors in both the simulated mean climate and the natural variability persist.
Due to the computational cost associated with the requirement of a well-resolved stratosphere, the models employed for the current assessment do not generally include the QBO.
In short, most AOGCMs do not simulate the spatial or intra-seasonal variation of monsoon precipitation accurately.
For models to simulate accurately the seasonally varying pattern of precipitation, they must correctly simulate a number of processes (e.g., evapotranspiration, condensation, transport) that are difficult to evaluate at a global scale.
This suggests that ongoing improvements in model formulation driven primarily by the needs of weather forecasting may lead also to more reliable climate predictions.
The spatial resolution of the coupled ocean-atmosphere models used in the IPCC assessment is generally not high enough to resolve tropical cyclones, and especially to simulate their intensity.
Many won’t understand the comments, which is exactly the point. Some of these alone are sufficient to invalidate the models. Collectively they’re a disaster, but illustrate the charge of Daylight Robbery. IPCC can say we told you about the problems; it isn’t our fault you don’t understand.
- From Dr Tim Ball, experienced Climate Scientist and CAGW skeptic