Matt,
I am a great fan of the IPI PEMs and have used the PEM1 and Climate Notebook for years. I have promoted them on this list in the past. I believe it is the best data logger for museum use. They are quite consistent over a two year period and some of ours have gone for 4 years or longer before they require calibration. The new PEM 2 is my data logger of choice. 

The PEMs are more expensive than the HOBO, but they in a completely different class and are worth it, especially since the data can be downloaded directly to www.PEMdata.com  website for interpretation, comparison to other sites in the same or other buildings and to outdoor data, and long term storage. Fifteen major museums across the country are presently using the PEM2 and testing a new more powerful on-line database to store and analyze data directly downloaded from the PEMs. Go to the Image Permanence Institute web site for more information. IPI also has a recalibration service and stresses the importance of recalibration.
 
One thing that does worry me is that users often do not pay attention to the calibration requirements for data loggers. Just because they have a digital readout and download to a computer, everyone seemed to assume that the data is always accurate. This is not always the case. Like any other RH sensor, data loggers do drift out of calibration and they should be recalibrated every year or two. If periodically checked against a high-quality regularly calibrated psychrometer, recalibration can be stretched to 3 or even 4 years. But if you are not checking the data loggers against a regularly calibrated psychrometer, they should be recalibrated yearly.

That is not to say there is not a place for the Hobo's as Mark has pointed out. Just be sure that they are reading accurately, especially if you have used them for several years without recalibration. 

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