I'm working with webworks for Blackberry and phonegap for iPhone. Not sure whether the problem is with webworks or underpowered handsets but we've had to spend a lot of time ripping features out of the webworks app to get any kind of robustness or performance when running on the device. Static google maps and thumbnail photos had to go.
All webworks apps on a handset share the same pool of memory, about half of the memory available to the BB browser. Webworks also leaks memory. There's a thread a year old on the webworks forum about the memory leaks and no solution forthcoming from Blackberry, just "workarounds". We had to alter the webworks sdk to make the garbage collection more aggressive to stop the app running out of memory all the time. But if other webworks apps are running on the handset and haven't had this GC tweak you will still get grief with memory.
Don't know if phonegap is any better than this but it'd have to go some to be worse than webworks.
If you want to do a webworks app keep the design simple, package all the graphics and assets in the deliverable (we were trying to get icons from an api but have ended up storing them base64 encoded in localStorage) and get it onto a handset asap so you don't waste time adding features that the handset can't cope with.
To sum up, webworks is pretty poor.