Some things to think about
pro: you're actually making _more than 10%_ annual return, assuming that they are taking money from your paycheck to cover the ESPP
con: think about the tax headache. Figure out how much you'd actually make annually, and decide if the additional return over just buying the stock (or something else!) is worth it. Also, assuming you're going to sell as soon as possible, there's some risk that the stock (or the whole market) would go down between the day that you purchase and the day that you can actually sell.
overexposure anecdote: I've seen people get wiped out from being overexposed. I've known people who were paper millionaires a couple times over due to options and company stock. Bad news from their company and they lost a bunch. One guy went all in, buying in his 401k + options + ESPP + brokerage. That stock (which was a very large, high-flying, somewhat well known tech company) went to single digits and he lost practically everything.
The rule of thumb is 5%. You have to judge the risk/reward ratio.