@Rino I., YES!

As any real Star Trek fan would know, the Ferengi 1st rule of acquisition states "Once you have their money, NEVER give it back!" lol

Well, net cash-flow is in your pocket unlike market appreciation or mortgage pay-down that is locked inside equity in the property. So, once its in your pocket you don't have to give it back (except maybe a small cut to the tax-man). 

What about if the property is an older property.. that needs work? such as new roof , plumbing ?

 Net cash flow is good, but in your plan you should have money put aside for capital expenditures. Those include plumbing, new roof, heating, etc.

After you have put aside money for those expenses, then the rest (barring you have paid everything else off) is net cash flow. And, like Kevin said, it is always good!

I would agree that net cash flow is great to have, and a requirement for properties I underwrite. You do need to account for capital expenditures such as roof, parking lot, plumbing, HVAC, etc. These are large expenses that you should collect and save money for over time. I also account for routine maintenance that will be needed and unit turnovers. 

@Rino I., on the surface, yes.

But, I guess the point of your question is either:
How do I calculate net cash flow? or
Is there a scenario that cash flow is actually bad?

On the first, there is no set in stone way.  You could look at net cash flow of the month/quarter/year/etc.  In this case, you have rents in, mortgage and repairs out, and possibly a new roof in that period.  That could be negative cash flow, but your roof is also a once-every-25-years expense.  A lot of people reserve for that outside of "cash flow".

On the second, even if you have true positive cash flow.  Every year, you are bringing in, say, $2,400/yr in actual profit after all your reserves, it could still be bad, sort of.  Let's say you bought in a marginal area where crime keeps getting worse.  Other home owners are highly leveraged with bad credit and no savings.  Your property value is dropping $10,000/yr on average.  In this case, you are cash flow positive, but actually losing money (unless you hold forever and don't worry about the actual value of your property).  While this is an extreme scenario, I would argue positive cash flow is not a good thing here.  

I think you are wondering if net positive cash flow is better than zero or net negative. Of course. But making $1 a year in cash flow is not great. Especially since cash flow is calculated before investor payback and income taxes. It can be bad if the net cash flow came about because of deferring maintenance on the property.