I just picked up a Gaggia Classic and I am wondering if my delicious Denver tap water will do the trick or if I should go with something bottled. I am looking here for the espresso standards and here for the Denver water report.
Nearest I can tell, it's pretty close to the sweet spot for total dissolved solids but I don't know how to decipher the other values like pH.
I plan to descale every now and again so mostly I am just curious if this H2O will cause any serious pain.
Provided that you descale your machine whenever needed, your machine should not take damage from using tap water.
Since your tap water is also safe to drink, this leaves taste as the only reason for concern. Dealing with this question is simple: just make a direct comparison between bottled-water espresso and tap water espresso. Check if you taste any noticeable differences. If this is not the case, the tap water should be fine. While there may be "guidelines for taste" as far as the tap water vs. bottled water question is concerned (e.g., softer water is typically better), what tastes good and what does not is a very complex question - so the tasting is the only way to be sure.
Note that you will need to descale your machine from time to time in both cases - so using bottled water to avoid descaling is probably not cost-effective (nor environmental friendly).
I remember that I have read a 80 pages long report on the bottled waters in US for brewing coffee in an home type espresso machine. The answer is simply, there is no easy answer if you want to go into the details.
There are at least one hundred ions in drinkable water and each one affects the extraction process during brewing (chemically). So, the answer is the taste.
More importantly, in my opinion, you should take care about your gadgets. Please notice that tap water may cause calcification.
Expanding a bit more on my comment on another answer...
Besides taste of the water itself, the very nature of the water (e.g. minerals and other water factors like pH that was cited by OP) will have an effect on extraction also. That is, different "water" will actually extract differently. This is a marginally distinct from a simple sum-of tastes; i.e., taste-of-water plus taste-of-coffee is not necessarily the whole story.
As I suggested before, try the extreme of bottled water as your comparison: distilled water. Though not always recommended (for a variety of reasonable and/or dubious reasons; more below) it will be the most stark comparison you can make that is reasonably easy. Holding all other factors as constant as possible, brew with your tap water and with distilled water. See if you can taste any difference; I (personally) don't think I could, but I'll certainly try this also! Play around with it, and enjoy the delicious coffee you make while you experiment. :)
There's interesting (to me) debate about the use of distilled water (and, more broadly, "what is the best water to use for brewing coffee?"). To your original question, any water will work for brewing coffee, but there is debate about what is best. Some warn of "dangers" of using distilled water (e.g., metals leaching from the machine, such as in this discussion on Home-Barista) or some quasi-scientific stuff about partial pressures and minerals in this article from The Coffee Brewers.
Mineral water always works best you can't compare it to tap water, but it's really depends on the quality of the tap water there if it's good most likly you won't notice the difference.
