Haven't bought mine just yet, but I'm getting ready to. I've been researching this for almost 3 months. Tablo is very clear in all its website info that you WILL need extra stuff for its unit.
Typically, the first piece of hardware to fail in a DVR is the hard drive, so I'm not real concerned about needing an external HD for the unit. If the HD ever goes bad, it isn't a bank-breaker to get another.
Second, by using WiFi in my home, I don't have to split the antenna signal into multiple sites (I have 7 rooms in my home with TV access, but with digital cable have only opted to pay for 3 boxes since each box costs MORE than $5/month). Every split of the antenna's signal means signal loss, but with the Tablo I really only need one feed from the antenna. I already own two Apple TV units, and a Roku box isn't that big of a deal to me compared to the multiple HUNDRED $$$ of other household DVR systems.
Finally, while many will say that Amazon Prime, Netflix, and Hulu (whichever version of the day is now out there) should solve all your major network viewing needs, but they don't really carry local news, weather, and sports. Sure I can look at my iPad or SmartPhone for the info, but some old habits die pretty hard.
The $5 per month ($50 per year) software guide would simply replace one of my 3 current Cable boxes. So with an initial outlay of $300-$450, my payback period on my cable cutting is only 4 months. That's not a bad ROI!