Specifically his Lives of the Caesars. I've been told he is the primary secondary source (that just sounds weird, doesn't it?) on Caligula and Claudius, but can we really take his account of their reigns at anything near face value? Would someone like Cassius Dio or Josephus be better choices?
Personally, I like Suetonius, but you have to know what you are getting into. A lot of his sources, especially for the emperors there were before his time, were just rumor and "popular history," if you want to call it that.
My personal opinion (as someone who studies Roman emperors, especially in the 1st century) is that his "unreliableness" has been overstated. For instance, Suetonius mentions a "revolving dining room" owned by Nero, which for a long time was considered just another Suetonian story. However it was recently found, which makes you wonder what else might be true.
Basically, take everything he writes with a grain of salt, but if it can't be outright dis-proven, I would consider most of what he says as possible (unless it is just ridiculous, which some of it is). We're working on a degree of guess work as it is.