It works very well with the gdb debugger, which was nice. I had problems with large old-style Fortran programs when compiling with NAGWare F95 and turning optimization on. These same programs ran with no problem when compiled and optimized with f2c+gcc. Thus, I would not recommend it as a Fortran-77 compiler; f2c does just as much and seems to be more reliable. NAGWare F95 may be fine for f95 work.
More details are available from NAG's web site (US users click here for the faster, US mirror). In particular see their page on F95 compiler products (faster US mirror).