As noted by the other thread, I do capoeira.
Overall, I've practiced capoeira, kendo, TKD, aikido and hap ki do. All of them seriously, but none of those to any real degree of proficiency. (I have a bad habit of quitting into the second year)
Can you give us some comparative insights? Similarities, differences, advantages, disadvantages.. fitness levels, learning curve for new people etc ?
The two taht stand out the most for me are Aikido and capoeira
Aikido is the gentle art. More gentle than judo, juijitsu, but still packs a punch. There are few strikes of any kind (in some forms of Aikido there are none at all) and mostly focus on proper movement rather than strength. The whole point is to move from your center of energy so that you expend as little of your own energy as possible. If your energy is strong enough, you can throw without touching a person. (You're not throwing them through the air like telekenises, but you can make them fall. I once saw a demo where my sensei, George Simcox, R.I.P., drop an attacker by bowing at him). Strength is almost a detrement in training becuase if you get used to throwing without leverage, center, or ki if and when you lose that strength you've also lost your aboility to perform the art. It's "maturation" time is far longer than other martial arts and you can expect to gain [black belt] rank at a good school in about 10 to 15 years of study. It's a great compliment to many other martial arts out there.
Capoeira is a very interesting sport with a very rich history, and requires the most physical training of all the martial arts I've done. In play there is no contact and is very jovial. There is a great sense of community and the people are very freindly and inclusive. It's often said that capoeira isn't good for self defense but I'll attest that the claim is a load of hogwash. I've seen my maestre get into a fight and knock a guy out (at a party that went sour.) Also, in the 1970 gangs trained in capoeira so heavily that police members had a hard time against them trying to fight back against such well trained combatants. The police eventually had to start training themselves in capoeira to be able to fight back. This martial art is very romanticized for it's flips and acrobatics, which is what necesitates the heavy training. Expect to sweat and expect strong glutes. Train your upper body particularly your whole back, abs, sides, triceps and shoulders. You'll get proficient quickly, and will love playing, but getting higher in the ranks may take some time. Keep playing in the roda (sparring) and you'll do fine.
Kendo is very fun and is a sport/ competition art. About the only self defense you'll get out of it is learning to hit people quickly with a stick and a general combat sense (you won't blank out in the fight.) Heavy cardio if done right. A lot of emphasis on proper form. Lighting fast when done right.
Tae kwon Do has two versions. The McDojo version (one at every strip mall) and hardcore sport version. If the people practicing have floppy limbs, lose balance easily, look out of shape or breath while doing it, chances are you're in the McDojo (you'd you like fries with your blackbelt?) The harcore version trains you hard. Lot's of emphasis on form and are anything but sloppy. They bounce while sparring and when they kick you can tell they mean it. I used to practice with a Korean grandmaster and afterward some crazy Jamaican guys. (Jamaicans and Morrocans are intense in their martial arts.) You'll learn fast in this sport with a good instructor. 5 years to a black belt on average. Good for self defense after ALOT of practice. Many stunt actors and Wuxia actors are TKD trained.
I did Hap Ki Do before TKD. It's alot like aikido with short range kicks thrown in. Where it differs from aikido is that it's anything but gentle on the opponent. You'll see kicks, breaks, pins, strikes, holds, take downs throws and the like. It's been a while since I've done this so I've forgotten much of it.
In general, stretch ALOT for any martial art, learn to fall, and beware of white belts, their inexperience makes them more dangerous than black belts.
I hope this helped. Let me know if you have any other questions.