[NH.Solar]

If the roof is over ten years old we generally advise our clients to consider replacing it before installing an array. There are a lot of variables however that can change this the main two being the pitch and the quality level of the original shingles. The roof may look fine but over time the UV rays of the sun will bake the oils out of shingles and they become brittle. Walking on them while installing an array can cause fractures or breakage and could result in a leak. Often a client can get away with just doing the south plane under the array because that is the one that will have seen the most direct sun. Once the array has been installed the shingles below will never age because they are now shaded from the direct UV rays by the panels. An additional benefit that comes from changing out the shingles before an solar installation is that the cost of re-roofing the plane under the solar array can be added to the cost of the installation and becomes subject to the 26% Federal ITC. Removing and re-installing a solar array would cost thousands of dollars so not doing the job correctly up front is a false economy.
In my experience a properly done solar installation will usually add value to a home in direct proportion to its gross cost and can also increase the homes marketability. If there are two identical homes and house A is listed for 300K and has an average $200/monthly utility bill (~1000kWh/mo), and house B is listed for 320K and has a near to zero monthly utility cost, which do you think will sell first? (...a rhetorical question, it depends on the buyer). Remember too that if you as the seller are able to add the full gross cost to the selling price of the home and sell it, you are already ahead of the game because you will have already pocketed the ITC and PUC rebate. All of the homes we have installed solar on have sold quickly and easily. A leased solar system on the other hand is definitely a detriment to a homes value and marketability. In fact there is nothing good to be said for leased systems and that is why the trend died so quickly.
As FlyingScot pointed out the aesthetics on an installation are a key element. We use all black modules whenever possible because the array then looks more uniform and sleek. An additional benefit is that they shed snow a little better than silver framed modules.
Yes the tech is advancing constantly but I might differ with the term slowly. Every week we are seeing something new coming out, most are relatively minor advancements but every now and again there is something really substantial, especially in regards to home energy storage. So does that mean it is wise to hold off until the next great thing comes along, probably not. As FlyingScot again pointed out a system that meets your needs today will still probably still be fully meeting those same needs 30 or more years from now.