I picked up Systema because I like the idea of being able to work out without much fear of being hurt. I like being able to go to work the next day and still be sore, but "good sore". I think it's interesting that I'm more ready to defend myself after a class, as opposed to being too ruined to have the strength.
I like the idea of being able to train for years and be in better health instead of having worn joints or other subtle injuries from, say, shadow boxing. Ask practitioners about their shoulders and knees.. they're like war wounds which people are proud of. Wierd. I've talked with some guys who have had previous injuries greatly helped through their training. It's quite inspiring to work with someone who is much older than me and who is in fantastic shape.
The workout itself is interesting. Using bodyweight and slowing things down as opposed to repetition. Focusing on proper breathing and posture to make everything more effective. Staying relaxed and keeping movement economical to waste less energy.
Psychologically, I found a few benefits.
Without ranking I don't have to feel judgemental about where I am or what I know. I can work with anyone at any experience level, and have even trained with instructors without knowing they were instructors. A battle of egos ends up being more about how much one person can help another than what rank a person is or who can kick who's butt.
Freely working with multiple partners gives me exposure to different body types, personalities, preferences and skill levels. Big guys, little guys, women, people who like kicking a lot, people who like wrestling, people who are tense or relaxed or injured.
Without kata or rituals, I don't have to do a lot of memorization. My memory is spotty at best, and I find it very stressful to learn a lot when what I need is a good fun workout and some experience. Beginning with formlessness and training towards a kind of naturalness is also quite interesting.
Fun. Yes, training is fun. I can smile and laugh, and it's not a bad thing. Sounds small, but it's very important to me. I am recharged at the end of a class, physically and emotionally.
So of all the choices I read about, I ended up picking
Systema.