Not entirely true. On Tomcat 5 and 6, the resulting code is really close to the original, plus lots of out.write().

If the logs show an exception on org.apache.jsp.mypage_jsp._jspService(mypage_jsp.java:NNNN), you can find the generated Java class on TOMCAT_HOME/work/Catalina/localhost/myapp and it's rather straightforward from there.

I do this on a daily basis to maintain a legacy app. Not sexy but, not hard either.