The share scheme where I work is nowhere near that advantageous, but I'm still putting in ~£550 a month..
In our case the purchase period is every 6 months, at a 15% discount from the lowest of either the opening or closing price of the 6 month period. Of course you do pay income tax on the profit made at the point of purchase, which hurts a little (as it comes out of the paycheque immediately after purchase, whether you've sold the shares or not!).. e.g. if I buy at $68/share but the market value is $80 then I pay income tax (at 40%) on $12 per share.
Not too bad at the moment (the share price is low) but when we bought at $80 and the share price was $140, it stung a bit!
Right now I'm treating them as short term investments - as it seems you would be - and selling them as soon as I get them (the NASDAQ is going up & down like the proverbial whores drawers) so there is very little risk involved at all, financially. Unless the stock market implodes in the ~3 days it takes for the shares to vest and become sellable, of course..