Once you get your large image or map loaded, its wonderful! Its stored on your iPhone/Touch/Pad so you have it with you even without network access. Scrolling and zooming is as smooth as Google Maps.
The app works by rendering whatever image you give it at full size, broken into tiles, so that it can efficiently display the image, just as Google Maps does.
It does have a few rough edges, of course its FREE, but here are the issues Ive noted:
1. The interface for loading new maps is a bit clunky. (copy/paste is your friend to get your url in there) If for some reason the "processing" stage fails, sometimes the app will ask to resume processing, but other times it will just be stuck there forever, and you have to delete it to start over.
2. Older iPhones/Touches have very little RAM, so its hard for TransitMaps to process huge images, and it can be VERY slow on large ones given the limited computational capacity of the mobile devices. It would be great if the author could write a pre-processing app for Mac/PC, so you could put a pre-processed image on the web somewhere and download it straight into the app without making the iPhone compute all the tiles.
3. REALLY big images can be tricky. The filesize itself cant be more than ~100MB (maybe less) and the image dimensions cant exceed about 30 mega-pixels if its in color (thats roughly 5600x5600 pixels) or larger than that if its a grayscale or black/white image.
4. PDFs seem to work slightly faster than PNGs, and using an image editor to reduce to 256 colors (especially without dithering) can really help get the filesize and rendering times down. If its a map (solid colors, lines) you want to avoid JPG.
5. If you find some site that gives you an image but it doesnt give a filename with .pdf or .gif or .png etc, just save the image on your computer first and upload it to your own website or a photo sharing place like flickr or imgur. Or turn on web-sharing on your own computer and connect via your home lan.
I hope that since its free the author wont stop working on it. Its a fabulous app with some room to grow and improve in the future. Write to the author to express your support, and try out his other (paid) apps as well.