The PDA version is kind of hard to use since there is no index (unless I'm not using it correctly and just can't find the index). If you want to find something, you have to use the search function and basically click through the results until you find what you want. However, you can add bookmarks to the PDA version so you can easily find what you want if there are things you use regularly.
I got tired of carrying around the book during my 3rd year clerkship, so started just using my PDA, but I think the book format was easier to find things. I'm not sure what resource I'm going to use during my 4th year sub-I's. If you have the most recent version of HLH, it comes with the free software download, so you might as well check it out.