Conventional wisdom is to never hinge your retirement on the same company that employs you. If it were to go bankrupt then you'll have the double whammy of no retirement and being out of a job at the same time. Having the option to essentially purchase stock at 1/2 price is pretty bad *** but I'd only consider it as a source of extra income. The only way I'd use it for retirement planning is if I planned on it being a source of income that I didn't need at all and only considered a nice bonus. I'd rather buy it in an IRA that I rolled into a diversified fund sometime down the road. Either that or have it buy me a Corvette in 10 years.
401K--Yes, they are portable (by law) and the transfer fees are limited & reasonable last I checked. Also, many companies will continue hosting your 401k account if you have a minimum of $5000 invested. Dollar cost averaging has always been the go to method, but this has come under fire from the 1999 and 2008 bubbles both popping in such a short time period. The calculator I meantioned earlier says this method is still viable even in a worst case scenario where you started investing right at the prior peak, meaning your returns would have been the lowest possible.