I guess I'm what you call a "book learned / internet learned" home wheel builder (as opposed to having been mentored in a shop environment), and on my first half-dozen builds over many years, I got along just fine without a tension gauge. The most important factors in building a reliable wheel have proved to be sufficient, even tension. (It took me only one attempt to figure out how horribly unreliable an undertensioned wheel could be!) But... with that number of wheels to my name, I was always curious what tension I was ultimately stopping at. So I've had this meter at my side for maybe about five years now.
If you're the least bit neurotic or OCD, this tension meter will simultaneously be the best and worst friend in your toolkit. It's possible, for instance, to perform your initial lacing to a low tension value, rather than driving the nipples to where the spoke threads just disappear below the nipple. Possible, but a huge time waster.
It's possible to go around and around and around and around again, a quarter turn tighter here, a quarter turn looser there, in an attempt to achieve "perfect" uniform tension as indicated the the Park's scale. Again, a huge time waster.
Here's how I prefer to use the tension meter: leave it in the box initially. Lace the wheel, lightly tension, stress relieve the spokes, build the tension and true with care. All the while, pluck the spokes and listen to the tone. Pull the tension meter out to verify final tension, but then trust your ears while plucking your way around the wheel looking for noticeably high pitched (over tensioned) or low toned spokes.
This meter is great as a final check, and a neat and somewhat inexpensive toy in that respect, but when you're comparing successive spokes in a fully-built wheel, the plucked tone they generate is going to highlight the problem areas far more quickly and just as reliably as this meter.
In a nutshell: This works, and will give you a numeric value to tension to, but *won't* give you a more reliable wheel than what you're capable of building without it. So by all means, pick one up if you're curious and don't mind the investment, but don't add it on your "essential items" list, as it's not a substitute for care and patience and understanding, and definitely not a crutch for inexperience.