Why does this lounge have its own thread?
Unlike most non–United Club Star Alliance (*A) lounges in the United States, the Lufthansa lounge at IAD is physically accessible to domestic UA passengers. This means:

1. United *G members may access the lounge on domestic itineraries, even though they wouldn't be allowed free access to the United Clubs on such itineraries.
2. Passengers who otherwise have United Club access, whether due to a paid membership or a foreign *G card, have the option of a vastly superior lounge in IAD.

What is the Lufthansa Lounge at IAD like?
There are two floors to the lounge: the concourse level, where you enter and check in, which also has the Senator lounge with ample windows; and the lower level, which has the Business lounge and showers. (The showers are very nice; on par with Senator lounge showers in Lufthansa's Frankfurt hub.)

The Senator lounge offers a complimentary buffet throughout the day, including hot dishes, salads, and dry snacks, and has draught beer and a full open bar including champagne, available upon request.

[Need description of FTL offering. Include photos?]

How do I get there?
The lounge is located at the center of the B concourse, from where non-US airlines depart, to the right of the Aerotrain escalators as you enter the concourse from the train.

Hours have changed seasonally, but, as of April 2014, are 8:20 to 21:30 except Mondays, when the lounge closes at 17:30. It also closes daily from 12:00 to 13:30. Some report the lounge opening earlier on occasion and some report it closing earlier on occasion.

From TSA/check-in, the B concourse is one stop away by Aerotrain, but in the opposite direction of the C concourse from where most United flights depart. Allow 20 minutes between the center of the C concourse and the Lufthansa Lounge. The longest part of the trip is the walk between the C concourse proper and its Aerotrain stop. Between concourses B and C, the Aerotrain makes two stops: one at concourse A and one at the main terminal/TSA/baggage claim.

How do I get inside?
With...

• Any Star Gold (*G) card (including UA Premier Gold and higher) and a same-day departing *A boarding pass in any class to any destination, domestic or international. There are reports (April 2014) that the physical card is no longer necessary; a *G designation on the boarding pass reflected in the barcode may be sufficient. (Senator lounge, 1 guest)
• A LH/LX HON/SEN card and a same-day departing or arriving *A boarding pass to/from any destination, domestic or international. (Senator lounge, 1 guest [plus spouse if HON] plus children under 18)
• A LH/LX FTL card and a same-day departing *A boarding pass in any class to any destination, domestic or international. (Business lounge, no guests)
• A LH Premium Economy boarding pass with ($35 adult/$20 2-12yo child) lounge pass purchased at the ticket counter. [Can it be paid at the entrance?] (Business lounge, no guests)
• A United Club or Maple Leaf Lounge Worldwide membership card and a same-day departing *A boarding pass in any class to any destination, domestic or international. (Business lounge, 1 guest)
• A same-day international departing *A First (Senator lounge, 1 guest) or Business (Business lounge, no guests) boarding pass
• A same-day arriving LH/LX First boarding pass. (Senator lounge, 1 guest)

In all cases, each guest must also present a same-day *A boarding pass for admission.

Lounge access for travel in a domestic premium cabin is not regulated by *A; as such, UA F domestic passengers (without other credentials) do not have access to the LH Lounge according to *A.com, just as they do not have access to the United Clubs. However, the international requirement for C/F tickets is conspicuously absent from Lufthansa's own lounge access sheet (see below), and domestic UA F passengers have reported admission. (Senator lounge, 1 guest)
[If we get (semi-)official confirmation of this policy, we'll add this to the bullets.]

Provided the lounge does not get overcrowded, as is often the case on US-based LH lounges, you may exit and re-enter the lounge as often as you wish. Note that if you are entering on a lounge pass, which are issued by Lufthansa to their own elites for various reasons, re-entry might not be allowed.

(If your access question wasn't addressed by the above, first check the *A access policy and LH's official access policy [generally relying on the least restrictive policy of the two if they appear to conflict], and if they answer a question that this wiki couldn't, update this wiki to help others! And if they don't, ask in the thread!)

So, can I use the lounge upon arrival in IAD?
Only LH/LX SEN/HON (on any *A carrier) and LH/LX F passengers have access to the lounge upon arrival at IAD without regard to onward travel. See the list above for more details. There are reports, however, of passengers not meeting this criteria presenting their arriving boarding pass and still being admitted.

Furthermore, note that, by virtue of not having access to the terminal, no international passenger whose journey ends at IAD has access to the lounge; they will be ferried by mobile lounge to the International Arrivals Building, which deposits passengers landside. [Can pax without checked bags choose to clear CBP in concourse C regardless? Also, do arriving passengers on LH/LX/*A F/C connecting at IAD to Y-class flights have access? According to LH.com, pax arriving in C on LH/LX can use the lounges in Germany and at JFK upon arrival to freshen up. Is the same courtesy extended at IAD?]

How does the Senator lounge compare to the newest Senator lounges in FRA?
Not favorably. The Senator lounge is like the old FRA lounges (cramped, and before a LH flight departure, crowded). Perhaps due to the crowding, LH offers its departing F pax preboarding from the Senator lounge, something which the FRA/MUC FCT/FCL does not offer but is common among LH's outstation lounges. So, enjoying champagne and macadamias at the nose or upper deck while waiting is a nice feature of the IAD Senator lounge.

This sounds too good to be true!
Lufthansa agrees! If the rumors are true, as of January 1, 2015, LH's Business Lounges will become its *G lounges, which means non-LH *Gs will be directed downstairs to the Business lounge after that date. There's some discussion of exceptions, most notable being that *G traveling in longhaul business class will still be admitted to the SEN lounge. Lufthansa has been more generous than most *A airlines in admitting *Gs to its designated *A First class lounges—not even UA's own 1Ks get to visit United's first class lounges without a first class ticket—so this change will bring Lufthansa somewhat more in line with the rest of the alliance. Went to the LH IAD lounge in June 2016 with TK G and SK BP flying Y, we were directed into the Senator lounge.