What I like to ask our clients is; what are your intentions? If you plan on selling in the next 5 to 10 years, then I would say hold off. For no other reason than it is an expensive endeavor and the ROI may not be there. Also, why do you want the covered porch? Is it just to give you some shade? If that is the case perhaps a pergola or sails are a good option.
If it is truly to provide an environment where you can enjoy the comforts of inside your home while still enjoying the outdoors then I always recommend it. The options are limitless. We often recommend outdoor fireplaces, TV stations. A bar area for food prep, BBQing and of course enjoying those summer drinks.
At the end of the day, a covered porch should really be looked at as an extension. The engineering involved is substantial and you want a product delivery that will maximize itself in value.