Trainer alone is like death, yes that's true.
Trainer setup with trainerroad, videos (either movies or sufferfest), wireless headphones, huge fan = awesome training.
Not as awesome as outside in terms of pure fun for sure, but can be even better than outdoors for those dedicated to improvement. With Trainerroad, you can get objective, reproducible TSS scores for all your workouts and track your FTP with FTP tests to confirm. No cars, no distractions, just pure, quality workouts, every time.
It does take a bit of getting used to when you start if you're used to otherwise riding outdoors, but once you get used to being indoors and hammering, it's really high yield in terms of performance and time efficiency (no coasting).
It's so high yield for me that since I'm not a criterium or tactical bike racer (I do time trials and triathlons now), I am going almost all indoors this winter-spring and based upon current improvements, I do expect to
PR the bike splits come my late spring races.