I'm an Alaska Gold, United Silver, and I'm totally happy with JetBlue's Mosaic program. Mosaic's benefit is consistency, flexibility, and managed expectations - I know exactly what I'm going to get on a B6 flight instead of playing the upgrade lottery between sitting in the front vs. the last row middle.
1. No Change fees
The money saved from this alone offsets the nominal EMS upgrade cost compared to United.
2. Super flexible SDC routing rules
This is a killer feature, and JetBlue still allows you to same-day-change into city pairs, which Alaska did away with and United and AA doesn't allow- even at the highest tiers. This means I can book into San Jose but switch to SFO if my plans change, or even do LGA connecting through Boston.
3. Guaranteed EMS upgrades
On a 6 hour flight, and paying the equivalent of $14 to guarantee the seat I want rather than play roulette is something I'm happy to do. (Row 6 ftw.) Hell, that's cheaper than the cost of WiFi flying literally anyone else. Even though I get free upgrades to F on Alaska, I usually take B6 because I know exactly what I'm getting + the flexibility to change.
4. Free Drinks
Mosaics getting free drinks regardless of where I sit is doubly nice.
5. Free cancellation at any time on award tickets
You know who can't do this? Delta flyers.
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Mint is an awesome product, but Mint's whole value proposition is that it's something -worth- paying for. That's why it's so much nicer than (and usually priced far more fairly) than PS, DL1, or AA's flagship Biz/F, to say nothing of Alaska F. Hell Mint tickets start at $399 on transcon routes, which is cheaper than the Alaska MVP Gold upgrade pricing.