@Thomas Hickey The infuriating key to using and understanding Cap Rates is locked up in a combination of these threads.
First, as many have mentioned, they fluctuate. (HINT: So does the Market!)
Second, as @Ola Dantis has mentioned, you actually use Cap Rates the opposite of the way that most people want to use them. You don't just have NOI, and divide by a rate-- how are you going to know what rate to use? You use known market values and divide by known NOI to get the rates, then apply those rates to your proposed purchase (or rehab, or development.) Yes, lots of Brokers and Analysts know the 'going rate' off the top of their heads, but that's only because they're embedded in the processes of all four activities: leasing, buying, selling, and financing in those particular markets.
I'll also draw your attention back to the key that @Russell Brazil mentioned with one quick edit: He said, however a completely vacant building has a zero cap rate...and that does not make it worthless. This is very true, but easier to understand in the formula if one says
...however a completely vacant building has a zero NOI...and that does not make it worthless.
These are low, simple round numbers, just for show. Let's take a NOI of 10,000 and divide it by a cap rate of .1 ("a Ten-cap") ; we get a value of $100K.
Ok, so if a 10 Cap reflects a reasonably stable market in Springfield, AnyState, USA, the property's worth 100K. 4 years later, Mom & Pop have the same tenants, haven't raised the rents, and the market goes out of control. Now when we divide it by a Cap of .04, ("a 4-cap") we get $250K. Someone's going to be willing to pay $250K for this property and its $10K income (...or investment, or .."opportunity" as we all like to describe it, here.)
Mom & Pop did nothing. The Market did it all, and the market increased the "Value" of this property 150-fold. But no one thinks about Cap Rates at this moment when they buy & sell... a "son" enters the picture and thinks "I'm going to get 300 for this place!" and puts it on the market at $300K. When someone bites, they then reset the cap rates in that market to a 3.33 Cap (!)
Bottom line: The NOI Describes the property's operating expedience; the CAP RATE Describes the market, and from that comes value. (Yes, there are tweaks for quality and obsolescence and deferred maintenance, and some for location, but generally location describes your market and its subsets... and yes, you need to understand a market's traditional Cap rate to know what's up and what's down... ) But for general purposes, I'll repeat it and suggest you file this somewhere in your matters: The NOI Describes the property; the Rate describes the market.
In a super-heated market, if & when you're willing to consider paying the premium, you need to look carefully at that NOI, because small tweaks there, mean small adjustments to value in a stable market, whereas sometimes small tweaks to the Market (AKA the Going Cap Rate ) can make or break you. If you know you can double your NOI, then you're likely safe, but if you can only find upward rental increases of 10%, then look out.
When you buy the property under a 4 cap, and you you've improved operations significantly but you can't refinance in 2 years -- not because the economy's bad or because your tenants are losing their jobs, but just because the market then reflects a more-normal 8 cap and you lost 100K of value-- in market interest alone-- that's when lots of Owners go mad.
Run the numbers in every scenario, improved, unimproved, refurbished, as-is, Low Income, Market rent, Section 8, run EVERYTHING. This is the moment in history when all the DD matters.
Good luck.