[time-nuts] rubidium standard caveats?

[Christopher Hoover]

hi folks.
 
i intend to buy a used rubidium standard (10MHz preferrably), probably off
of e-bay.   
 
a few questions for you gurus:  any models that are especially winning?  any
models i should avoid?  other caveats?
 
thanks,
christopher.
 
ch<<<<at>>>>murgatroid<<<<dot>>>>com