Once you get over the pilot it's about 5.5- 6 out of 10 The pilot way heavy on the cliches & stereotypes ("he's old" joke every 2 minutes, sometimes less). It's a recurring motif throw out the series. ABC played it safe casting Fillion as lead, if you are a Fillion fan you will most likely be watching. I was never a fan of his, so his presents it somewhat distracting, it would have been more entertaining to see a fresh face, an unknown actor in 40-50's for this.
But here is the weird part: Fillion fans don't like it because he "deserved better" (come on people, he can't play the 35-ish charmer anymore) and as far as I concern- it's a decent casting for his age (being playful with the age issue). BUT- Fillion looks bloated. Too much nip and tucked, yes, Fillion is skinny again (he got fatter as Castle progressed) but looks bad.
The show itself has decent action scenes but many plot holes or unbelievable things in it. There isn't much currently on TV that I like (The rookie comes on ABC's Forever time slot- now that was an unjustified cancellation!), this show is O.K-ish. Will I be watching an episode every week? no. Maybe keep track of it every couple of weeks.