I get a 15% discount, looking back 2 years. I use ESPP as a risk-free investment; the risk of holding the stock for a year is too great for me despite the tax benefits (this is a controversial subject among my co-workers!).
Here's my reasoning (I used your 5% discount in the calculation).
Say I contribute $10k/year.
Stock's market value is $25, so purchase price is $23.75 ($25*.95), which buys 421 shares ($10,000/23.75)
421 shares are worth $10,526 ($25*421), so you have a short term gain of $526
Probably your marginal tax rate is lower than mine, but I pay 45.1% (28% federal, 9.3% state, 7.8% payroll), so short term capital gains comes to $237. After tax free money of $289, yay!
Assume you hold for 1 year, so you get long term capital gains treatment. For me, this gives a marginal rate of 24.3% (15% federal, 9.3% state). If the stock price stays the same, I would pay just $128 in tax, saving $109.
HOWEVER, that whole year I would have the $10,526 invested in my companies stock. So if the stock price dips to $20 (an equivalent drop has happened in the past year at my company), then my 421 shares are only worth $8421. Add in tax benefit $384 for taking the loss, then I have a loss of $1195 ($8421-10,000+384). It doesn't take an Enron-style catastrophe to completely wipe out the ESPP gain and some of the principal investment.
For me, saving $109 is not worth putting the entire $10k at risk. Though it depends on your investment strategy as well. Personally I don't invest in individual stocks (in my company or otherwise), so holding any individual stock for a year runs counter to my diversification plan. And the tax benefits are too small for me to do that.