I too use the PRP merriam-webster collegiate dictionary, probably the best $25 i've spent on palm software.
of course, i bought it with the PDM gift certificate my parents bought me for christmas last year but, whatever.
Anyway, it's awesome. it has every word i've ever needed to look up, and it's faster than i'd expected. I'm thinking that PDM sped up the PRP dictionary engine altogether, simply because the mid-sized dictionary i had when PRP came out was a little slower than i liked, but now lookups are speedy enough for me (i'm running a 33mhz clie), so that makes me happy. but, yeah, it's great. highly recommended all around.