The stock purchase plan looks good to me. The one I used to have didn't have a match, but we got a 15% discount off the lowest price of the stock at the start or end of the six-month period (whichever was lower), so if it was $100 a share at the start of the period and $150 at the end, we got our shares at $85. Clearly, a no-brainer to participate in, with a guaranteed return of at least 15% and possibly quite a bit more. Often, I doubled my money.
It's not clear if you're actually purchasing shares with each paycheck or at the end of the six-month period, but I would definitely max it out in either case.
What you'll have to decide and think hard on is how and when to sell the shares. You could:
1. Sell immediately and lock in the 25% discount/return, and invest in an index fund or something else. I'd say it's the conservative choice.
2. Hold for the long haul. It can be extremely profitable, as evidenced by the returns you noted. However, it's also the riskiest because you have so much of your money in one stock/company (as well as your paycheck).
3. Once you have the stock, set a limit order to sell if it goes below a certain price that you're comfortable with. This way, you can ride the stock up and keep changing/re-setting the limit order, so you have all upside and no downside.
On the surface, #3 looks like the best play, although you could sell immediately and do the same thing with any other stock or mutual fund, and that way, you're diversified better.
I guess a fourth option would be to keep some and sell some.
And, I forget what the tax implications are of selling right away vs. holding. Is it taxed as income or capital gains?
Anyway, I recommend you do it, and figure out your strategy, which can change. I did #1-3 at various times.