I just installed and tried all the chmviewer I heard of. Here are some of my views of the software I tested:
Chmreader.v2.0(Aroy Ltd): Price $9.95. Even though they says it supports Windows Mobile 2003. But many of the users complaing it is not. It installed fine on my x51v. But when I open some chm books, it shows blank pages in all of them. I feel pity for them whoever spend money on this.
Microolap chm ebook reader: Price $14.95(now $11.96) . After reading the description on their web pages my expectation was high on this. But I was disappointed. same blank screen for the books on my x51v. Maybe it could work for others, but not woring for me. Try the demo first before even thinking of buying.
Team Ones Reader 3 : Simple one line sentence for it: not worth at all. Horrible user interface. I didn't even think twice before uninstalling.
GowerPoint uBook 0.9b: Second best choice for me. Can read HTML, TXT, RTF, PDB, PRC. I was almost impressed by the output it produces. Then suddenly I noticed it does not display the embedded images in chm and pdfs. So there it goes to uninstallation queue.
CHMReader.NET 2.0.14: This is the only free chm viewer I have tried. Personally I don't expect much from a free software. But wow. It was amazing piece of software. It is the only chmviewer that supported all my purposes. It works on WM5/X51v. It shows index. It shows embedded images. You can search on the chm file. The only down side is that it does not support landscape view. Also opening large chm file takes some initial time. but hey, at least it can open. Many professional chm viewers even cannot open large files. And above all you are getting it free. I am sticking to it till I get a better viewer. You can get it here:
http://pjt.xanetuk.com/
Hopefully, this reviews will save some of your time to chose a better chm viewer.