*Spoiler Alert*I did watch it yesterday, and I was pretty impressed. It was well acted, and the script was well-written. I'm FTM, not MTF, but I didn't find it depressing: I'm also in my 60's, and I liked that Maura is an older trans person -- we're not a group that gets much media attention or general public awareness, so to me, something that says "Yes, older people can and do transition" is all to the good.
As to the choice of a cis male actor to play Maura, sure, it would have been more "politically correct" to cast a trans woman, but I don't know that it would have improved the quality of the show. Given that the show (aside from flashbacks) is set in a present in which Maura is just beginning her transition, casting someone who was a few years into hers might have made it less realistic, not more so. I thought Jeffrey Tambour did a remarkable and sensitive job of portraying someone whose physical appearance was completely at odds with her inner self.
Is the content unrealistic or depressing? Nah, not really. I think the way Maura is contrasted with her kids and wife, who are all somewhere between "major jerk" and "sociopath," makes it easier for a wide audience to find her sympathetic. She comes across as thoughtful, loving, and vulnerable, but also as someone who is pretty resilient. It's actually easier to believe that she'll be OK than that her kids will.
**The only thing I found a bit unrealistic was the ease with which Shelly, Maura's ex-wife, and their kids, decided to dispose of Shelly's husband Ed. I mean, jeez...