Doing this makes you retain all the advantages of running your own MTA: none of your emails are hosted at a third party provider, no scanning of your emails to personalize ads, no government agency can knock at the door of an email provider and ask them for the content of your inbox, etc.
The only downside is that in theory Google can scan and block your outgoing emails (not incoming emails since these hit your MTA directly). But if you don't send spam, this should never happen.
Another option is to route your mail through your ISP's MTA. Yes ISPs usually offer SMTP relay service accessible only from their customer's IP addresses (eg. for Comcast it is "smtp.comcast.net" IIRC.) However the reputation factor of an ISP's MTA might be worse than Google's MTA.
Are you an EC2 user, well there is a black list for those IP's.
Are you on a residential connection or a dynamic IP... there is a black list for that also. So you may have a perfectly configured MTA with a pristine IP and still have delivery issues. Using a smart host is perfect in these situations.
It's been mentioned before but part of running your own mail server is the process of learning and tweaking things to your liking. I remember the days when you could run a server on your own home connection on port 25 without any issues - but those days are long gone. A crucial part of every sys admins journey historically has been setting up and configuring a personal use mail server.
You can always run a mail server at home on a port other than 25 and have an upstream email service deliver it to your alternate port.
So if you'd rather not deal with filtering your own spam, or would like a excellent smart host with SPF and DKIM - let me know.
I'll provide commercial spam filtering with backup MX or smart hosting services to any HN user for 50% off from duocircle.com - just message me... running your own is sort of a right of passage.
I've smart-hosted using postfix, direct for incoming and gmail/sendgrid/mandrill (before it was neutered by mail chimp) for outgoing.
It was fairly cost effective - generous free tier (upto 12k emails) for outgoing, and a small vps to host postfix (~$5/month).
I personally want to control my input addresses.
Hosting your own you can get funky wildcard postboxen.
Hosting your own you can archive all the mail without 3rd party approval.
I can see the advantage of the small amount of outbound being via someone else.
The sad reality is, I'm thinking of moving my e-mail back to someone else. It might be Fastmail, might be Amazon. Running my own e-mail server is a pain and I hate having to send people a message on Facebook or Reddit saying, "I sent you an e-mail. Check your spam folder."
If I route my e-mail through Gmail's SMTP server as a relay, that shouldn't affect my DKIM/SPF stuff right? I'd just have to change SPF/DMARC to say gmail is allowed to relay messages for me, correct?
They do enjoy one big advantage over google besides privacy: tech support! Like a human, that reads and responds to issues submitted through their site! I've contacted them twice with questions about setting up my domain and one other thing and I got replies, from someone who knew what she/he was doing, within an hour each time!
Also, they will do a catch-all address (eg email@example.com) which receives email from every address not specifically defined. Thus you can implement the best anti-spam technique: every email you give out is firstname.lastname@example.org which forwards to email@example.com so you can tell exactly who sold or leaked your email address. eg firstname.lastname@example.org was sold all over the place.
I find it hard to believe that you get classified as spam for no good reason, unless perhaps that Linode / Digital Ocean are known for hosting so much garbage and people can be tempted to outright block them, but I doubt Gmail does that?