@Nicholas U. - a couple of other things to be aware of:
As other have said, there are better metrics than cap rate to base purchase decisions on. However, if you are thinking of buying and holding a property a specific period, like 5 years, then you need to estimate the value on exit and what cap rates might be in 5 years and estimating market value as NOI / Cap Rate in 5 Years is one way of doing that, so you need to understand what rates are in the market today and estimate what they'll be in the future if you're planning something other than buy and (indefinite) hold. So, you still need to understand them if you're buying multifamily.
Related point is about value creation. Let's say you're a value-add guy and you raise rents $10/U/mo on those 20 units or $2880/year. How much has market value of property gone up if it's a 5 cap property? a 10 cap property? If it's a 5 cap, increased value is approximately (assuming rents flow straight through to NOI) $2880/5% = $57,600 but if it's a 10 cap increased value is $2880/10% = $28,800. Wow...the market value of a $1 improvement in NOI on a 5 cap property is 2X that on a 10 cap property.
Finally, keep in mind that cap rates are pretty squish numbers in that sellers and buyers will calculate different cap rates on same property because their NOI assumptions differ. Let's say scheduled rents are $192,000 (20U @ $800/U/mo.) and seller claims expenses are $38,400 (20%) and NOI is said to be $153,600. A 20% expense ratio is likely artificially low and a (directionally) more realistic expense # is $76,800 (40%), giving NOI of $115,200. (There are several other refinements that should occur to preceding #s as well). Brokers will often quote cap rates based on pro forma calculation where they assume improved rents and reduced expenses and those cap rates will be greater than current because their pro forma NOI will always be better than the trailing NOI. ...Reminds me of "lies, darned lies, and cap rates"