Gamepass Ultimate is great value, especially if you're a PC gamer too, and can therefore benefit from both the console AND PC libraries, which do vary a bit.
I got a free trial for 3 months, and it was enough to convince me that the service, whilst not perfect in execution, is worth supporting as the closest thing yet we've gotten to a Netflix for games.
The Xbox Live Gold -> Ultimate upgrade route also made it almost rediculously affordable; using codes I paid about £117 for 3 years worth of XBL Gold, and then paid £1 to upgrade the entire lot to Gamepass Ultimate; it was actually cheaper for me to do this for both me and the wife; than it was for me to use codes for "Gamepass for PC" by itself for my wife.
So far I've played a load of Mechwarrior 5 on PC, literally worked my way through the single player campaigns of ALL the recent Battlefields except 2042, a decent bit of Battletech, a while of Back4Blood (although as above, I soon realised it wasn't fully cooked) along with dabbled on a number of other titles, and will likely do Crysis 1-3 soon too. The wife has also played Forza and a number of other games. I am also fully looking forward to doing a complete Halo 1-5 (inc Reach/ODST) Coop play through with the wife; and have also downloaded a few other games to the PC and to the Xbox. If we'd bought them all, we'd probably have ALREADY paid back what we both paid for 3 years of access, within just a few months, and that's excluding the fact we've both downloaded Total Warhammer 3 on PC to play with friends, that alone would have cost us both around £35-50 each right now. That level of value access cannot be denied, plus other games coming like the new Two Point University etc.
It is an ideal service, maybe not for games you will want to play LONG term, in which case an actual purchase is justifiable, but for the sort of games you will either dabble in and out of with no commitment, OR for games you will play through once and then never again (anyone with a substantial library; you will know this is actually a LOT of titles).
Yes, occasionally its frustrating that games I've already purchased land on the service; or that games that I could be playing can leave the service (which is a real shame as I missed a few I'd have played) but frankly that is a small price to pay for a service that genuinely has a LOT of content on, and that is slowly fixing the PC/download bugs that have plagued it. For years I turned my nose up at 'another subscription', but now that its had time to establish and build the library and maturity on both PC and XBOX, the value is genuinely compelling, especially for what worked out to around £3.50 a month...