Remember that yours is a business arrangement with an agent.
And that the agent, ultimately, works for you.
That said, then, "How's it going?" emails are going to get shoved to the bottom of their "to-do" list. They don't have time. You can socialize at writing conferences.
If your question is "How are the submissions going for (book title)?" then that's a legit business question.
If you've several questions, put them in one email.
If you just want to say "Hey, I finished 50 pages on the next book!" I'm sure he will be happy, but would rather you contact him with, "The new book is finished. If you're in the office I'll email the file. Just let me know."
He expects you to be writing, and he should be subbing your work to various houses.
My first agent was okay, but didn't keep me updated on things and arbitrarily decided whether I was right for a project or not. He put all his attention on his more famous writers and wanted me to write "blockbuster" books--which ain't my thing. When he promptly did the exact opposite when I told him not to call a publisher about a pending deal (and effed up the deal) I fired him. He was honestly puzzled.
My "new" agent of 12 years now is awesome and keeps me informed when she subs to foreign markets, sends links to reviews, and has a newsletter with guest blogs and runs interference when I have delivery problems. She sells the hell out of what I write and had landed deals I never expected.
So they're not created equal.
Finding the right agent is like finding the right life partner. If at any point you decide this guy just isn't a good fit, then you dissolve the partnership.
If one agent liked your work, so will another.
