This is a psychological question, not a business one.
I.e., only you can decide how important the future of the company is to you after you're gone.
I.e., if your conscience is going to make you miserable while you're sipping margaritas in Cancun, don't do it.
I.e., good deals are few and far between, and you may kick yourself (and your conscience) if another one doesn't come along.
I.e., you could die tomorrow and your successor might run your company into the ground; how will you prevent that?
No one can tell you (and your conscience) what is "right" or "wrong," but, personally, I'm a believer in TMR (take the money and run).
If guilt weighs too heavy, call those customers and offer to donate a chunk of your cash to their favorite charity. (kidding)

All good advice here. As Bruce says, this is more a psycological question than anything else.

It's very very commen that entrepreneur of company's they have built up feel a strong sense of loyaly toward them. But remember to think about your future/retirement here as well. I'm no private wealth expert by any means, but having all your chips as you put it, on one table is risky, particularly going into the later stages of life, so being a bit more diversified would reduce this risk.

Of course you probably have a very valid case about the customer base potentially being eroded if a competitor to them buys, and its nice to see a boss who has their employees futures in mind.

http://www.nequityonline.com

Manoj makes some very good points, and his advice is worth considering. However, there's also a potential downside to involvement post-sale: When you are only a title, or even a minority stockholder, you have no control over what the new company does. I know, that's obvious, but maybe not so obvious is the fact that the new owners may do things that you don't like; you will be powerless to make changes, and you may not want your name associated with their decisions. As one of my best mentors once told me, if you don't own more than 50%, you're just cannon fodder. I'd still take the money and run. :-)

IMO, the key drivers to selling include:

1. Are you tired of the company? More critically, are you tired of the industry? Do you want to move on and find something new? Maybe you've always thought that building a startup in (say) alternative energy or wholesale cabbage trading would be "cool" yet you've been stuck in your current vertical all these years.
2. Is the offer large enough that you have "FU Money"? As Scott Burns said in a recent blog posting: You are wealthy when more money won’t change where you live, what you eat, what you wear, what you drive, or who you sleep with.
3. Do you want to continue working at the company? If your goal is to bring in money to  sell a portion so you can diversify your holdings yet continue to run and grow the company... I'd run away quickly. Think about the next buyer when you want to sell the balance of the company. Said another way, selling 20% of the company today means you still own 80% yet the company with 20% ownership has way more control than you want.

My bottom line: Plan to sell 100%; I'll second Bruce's comment re: totally exit the company. If you can't "retire" or move to another industry once you've sold, think about the industry relationships you have and view the deal from your current customer's eyes. Once you sell the technology to a competitor, they will likely hate you and you'll be mentally blacklisted; don't plan to sell them anything at your next company. They will all say, "What have you done for me today butthead?" not "I enjoyed our 15 year relationship and the long list of personal sacrifices you made to give me real value all those years."