Cary and MTV both have good points. I'm "drinking the ADS-B Kool-Aid" because:
a) My airplane is based inside the Mode-C veil, so I have to have it after 1/1/2020 or park the airplane,
b) Most (90%+) of my flying is within 300 miles of Dallas, and FIS-B weather reception is great here, and
c) TIS-B traffic is a game-changer for avoiding airborne traffic.
The traffic information alone would justify the upgrade for me – in my flying area. Once while on a cross-country flight from Dallas to OK CIty (and well before the ADS-B upgrade to my plane) I was overtaken from behind by a Bonanza who passed about 30-50 feet below me, from my 7 o'clock to my 1 o'clock position. There was absolutely no way I could have seen him, and he obviously never saw me! After the ADS-B upgrade to my previous plane, I was amazed at all the traffic it revealed that was previously invisible. On several occasions, I was able to observe airplanes overtaking me from behind, and reduce the potential for conflict by altering course or altitude slightly.
Probably if I was flying "out west" in the mountains, XM would be a higher priority for me. But in about 3/4 of CONUS, the slight difference in resolution between XM and FIS-B "close-in" (<250 miles) is immaterial to my weather-avoidance purposes. Beyond 250 miles, XM's advantages are increased, because they continue to supply "regional" NEXRAD, while the FIS-B broadcasts only provide the "CONUS" data, which is definitely lower resolution, but in my plane, that's 2+ hours of flying time... To be fair, XM's product is superior even in the "up-close" NEXRAD views. XM delivers smaller "chunks" (pixels) of data, so things like "hook lines" are more defined than with FIS-B. But for me, if I'm close enough that the resolution difference really matters, I'd better be on the ground (preferably in a hangar!). There's nowhere I need to be badly enough to fly close to an active thunderstorm. (And even if I DID have to go, I'd rather have a Stormscope to show the lightning activity than just rely on 10-15 minute old NEXRAD data, as good and helpful as it is...)
Here in thunderstorm country, I use FIS-B as a "strategic decision-making tool" to avoid thunderstorms by at least 40 miles (target is 50, but my personal "go-no-closer" distance is 40 miles). That's DOUBLE the distance the AOPA and FAA used to recommend, but even they are now saying 20 miles isn't enough... That's partially because of the roughly 10-15 minute delay between what you can see out the window and when it will show up on the screen. But I've also grown a bit more conservative because of personal experience with outflow winds from an thunderstorm.
In my own "I Learned About Flying From That" incident, I got caught by "outflow" winds from a thunderstorm that was over 30 miles away. I was taking off into a 10-15 knot direct headwind, trying to depart well before the thunderstorm cell got any closer. I had rotated and climbed to about 50 feet, when I got hit by a really strong tailwind that brought me down to barely above stall speed, and forced me down to about 5 feet above the runway. Fortunately, there were no obstructions off the end of the runway, and I was able to lower the nose and fly in ground effect for a while to regain some airspeed and resume my climb. Since that incident, I give thunderstorms more clearance!
XM is a great product, but it costs money. Money that for my type (and area) of flying doesn't justify the $400+ yearly expense for the subscription, and the $500+ for the hardware to receive it. Once I've upgraded my plane for the 2020 mandate, an inexpensive (<$200 to maybe $800 tops) ADS-B receiver will give me first class traffic information, and 2nd-class weather information, with no subscription fee. To me, that's a good deal.
Pass the Kool-Aid, please!