Proper use of cvs

[csj]

I managed to download some source trees using cvs (five different 
programs in fact). None of them would compile successfully. Note that 
these are sources of programs whose latest ftp/http sources all 
compiled smoothly (give and take a one-line tweak). Any idea what I'm 
doing wrong? Or is cvs supposed to be a developers-only tool, off 
limits to mere mortal users?