I finally had a chance to work in this database today, and like what I see. I have a Findmypast subscription, so the opening screen may look different from what other users see.
1) Here is the home page:
2) At the bottom of the screen above, the link to "Explore 33 million NEW U.S. marriage records." I clicked on it to see the search page for this collection:
3) I entered "seaver" in the surname field, chose no name variants, and the search results included 2,311 entries:
4) I scrolled down a bit, and found an entry for an Archer W. Seaver who married a Lillian in 1923 in Texas. I checked my RootsMagic database, and I did not have a marriage date for them, so I clicked on the blue "transcription" icon on the right side of the page, and saw the indexed information:
Archer W. Seaver, age 22, married Lillian Carlson, age 19, on 29 August 1923 in El Paso County, Texas. Some entries provide the parents names on the record.
5) There was a blue icon to see the record image on the transcription page, so I clicked on that and saw the marriage license and marriage return:
The marriage license provided the birth date and birth place of both parties, plus signatures.
6) Not every entry in the U.S. Marriage Collection has an image of a license or return, but they all have a transcription of the indexed information.
7) A source citation? Here is one for this marriage record:
"United States Marriages," indexed database, Findmypast (http://www.findmypast.com : accessed 12 February 2016), El Paso County, Texas, Archer W. Seaver and Lillian Carlson marriage entry, 1923; citing original data from El Paso County, Texas vital records, provided on FHL US/CAN Microfilm 25,067.
8) At present, there are 33 million records in this collection, and eventually it will be 100 million records or more. There is often more than one entry for a specific marriage.
I have over 2,300 entries just for Seaver alone to mine in this collection. Some of them are in other record collections on FamilySearch and Ancestry.com, but there are quite a few I haven't seen before.