Pick guard replacement: A purely cosmetic upgrade of an arguably necessary part. Martin went to all black pick guards some time in the late 1960s. Why, I do not know, but in recent years many guitarists pined for the faux tortoise shell guards used from the 1930s until that point in time. So Martin began putting them back on all their mid to upper level instruments, and the boutique makers followed suit, as did virtually all the makers of imported guitars. My own tastes run toward those type of guards but I would not be adverse to owning, say, a 1969 D-28 with a black guard. Some Martin owners really, really hate the "pixilated" look of some of the new guards - i.e., if you look very closely you can see individual dots in the guards, the result of being generated via some sort of computer generated production process. Some guards are downright ugly in my book, such as the "tiger stripe" ones used on some Gibsons and the pale amber with dark brown splotches type used on certain imported models. I would definitely replace that one if I bought a guitar that had one! The replacement process can be kind of scary as it involves using a hair dryer to loosen the glue holding on the old one and the possibility that the new guard won't quite match the footprint of the old one. Bottom line - I would only do this job out of desperation. Rating: 2 (at best!)
Bone saddle & nut: This is the single best way I know to improve the sound of virtually any acoustic guitar. You notice I group nut and saddle together. This is because I don't think you realize the benefit if you only do one. This is a job best left to a luthier and depending on the materials used (bone of various animals, even fossilized bone or tusk) it can get kind of pricey. However, if I plan on owning a guitar for a while I view this job as an absolute necessity and I think you will too once you try it. Rating: 5
Bone, Ebony or Tusq bridge pins: If your guitar has plastic pins, this is an inexpensive way to get somewhat better tone. Best done in conjunction with the above mentioned bone nut and saddle. Just be sure to order the correct size. I have some nice pins available in my store.
Tuning machines: This is one of the most inconsistent aspects of guitar construction. I am constantly amazed by playing some very nice instruments that come through with mediocre machines and some inexpensive ones that have nice machines. If you're not satisfied with the tuning function of your guitar, this is definitely a good investment. Some of the good names are Grover, Gotoh, Schaller and Waverly. It is a job that you can so yourself with some patience, care and good tools. Rating: 4
Strap button on neck heel: Most new guitars come through with one already and every guitar I've owned for the last 15 years or so has or had one. However, there is a lot of debate about whether or not this affects the value. One well respected store owner in Pennsylvania won't even look at a used guitar that has one. But my guess is that he is in a small minority. If I had a 1950 Gibson or Martin that didn't have one, I'd think twice about doing it - that tiny hole that needs to be drilled will most certainly affect the value of a true vintage, collectible guitar. In the real world of playing though being able to attach the strap at that point rather than up at the head is a no-brainer. Rating: 4
Custom inlay: I once had a very nice 1970 D-28 that I wanted to make unique and recognizable should it ever be stolen so I had diamond and snowflake inlays done on the neck (side note: if you live in New England and ever happen to see that guitar for sale, please contact me! I would dearly love to get that one back, mostly for sentimental reasons). Not many people do custom inlays on their guitars but generally I do think it adversely affects the value of a higher end instrument. Your call on this - if you like the way it looks, who cares? Rating: 3
A better case: Although not technically an upgrade to your guitar, this is a great idea if the one you have is in bad repair or you're only using a gig bag. Not only will your guitar be better protected but this is one upgrade that almost always adds to the overall value of your guitar, assuming it's included in any sale. Rating: 5
Electric pick-up: Another of the options that are disdained by some but viewed as a necessity by others. I tend to break it down into vintage vs. non-vintage guitars. No matter how much I wanted to use it in a professional setting, I would never add a pick-up to a pre-1970 Martin, Gibson or Guild. Doing so would surely make those guitars lose value, even if I used the fanciest and priciest pick-up on the market. However, I pretty much always have a K&K Pure Western mini (the BEST sounding passive pick-up on the market!) put in any guitar I'm going to gig with if that guitar is fairly recent. Part of it has to do with the wonderful, pure and true sound I get from the K&K and a decent little tube pre-amp but also because using a pick-up with an acoustic allows much, much more control over my sound when I'm playing out. Playing into a mic is just too restrictive and prone to feedback. Rating: 4.9 (assuming we're talking about a non-vintage guitar)
So there you are. Obviously, these are only my opinions but they are opinions based on better than 40 years of owning guitars.
Peace & good music,