Okay, a lot of good comments here, some ridiculous.
Aikido practicioners or Aikidoists (Please avoid Aikidoka as a term for Aikido practitioners, technically, in Japan, it only refers to a notable, or high ranking, distinguished Aikidoist, for most people it doesn't matter, but I used the term once with a visiting Shihan from Japan, and he wasn't amused, he took me aside to explain the difference after class) get a fair amount of criticism for a variety of reasons from the MMA community. It goes back to a couple of fundamental differences between competitive MA and Aikido.
#1 Aikido is NOT competitive. The techniques were actually derivatives of techniques developed on Japan's feudal battlefields centuries ago. While it is a Gendai art, it's parent art is Daito Ryu (which is Koryu, although questions have been raised about that as well) as well as multiple other arts (O'Sensei also studied in no particular order Kito Ryu, Goto Ha Yagyu Ryu, Judo, Shikage Ryu, and IIRC, Tenjin Shinyo Ryu) which all played a role in the development of Aikido. Battlefield combat is quite a bit different than competitive "ring" sports. On the battlefield, you would often attack full on in order to try and win and deal with the next person. This is substantially different from an attacker who is probing, and focused on only one person, and trying to win. Just different contexts.
#2 Aikido has unrealistic attacks. The attacks in Aikido are all based on sword motions, or how you would move and cut with a sword....without the sword. While many people deride this as unrealistic, there are only so many ways to grab, punch, or strike someone...the attacks do look silly, but at some point in your Aikido journey you make an astonishing discovery.....that is....The attack DOES NOT MATTER...to be honest, I don't care how you attack me, your energy can only be directed at me in so many ways. The only thing the attack does, is potentially change my entry..but otherwise, I honestly don't care.
#3 Aikido does not pressure test. We actually do, with randori, jiyu waza, and other drills, which can be similar to "rolling" in BJJ, but it doesn't look that way. This is actually one criticism that likely has a little merit, but it can vary considerably by dojo...some are quite martial, and you will be pressure tested, and others are more soft, internal, and spiritual. I cannot speak for them.
#4 Aikido looks like a dance. Yes, it certainly can, but the point is.....ukemi is designed first and foremost, to protect uke. So, at higher levels, your ukemi becomes quite proficient, and it can look more dancelike, and a little less realistic. Trust me. Those same techniques executed on someone without proficient ukemi will badly injure them. I tell new students all the time that want to just learn techniques, that you have to focus on learning ukemi too. For example, I cannot do a gensake otoshi to you if you don't know the ukemi, which for that technique is really scary and pretty advanced. So, you have to focus on ukemi in order to progress as well. Most injuries that happen in Aikido, almost always happen because a nage is throwing or executing a technique on an uke who either doesn't know, or cannot properly execute the ukemi.
I practice BJJ as well as Aikido. Aikido is, and will always be, my primary art. Practice what you like. Don't worry about what others think. Their opinion is really not important or relevant at all.
BTW, I was just at a seminar this weekend with an 80 y/o Japanese Shihan who weighed maybe 120 pounds. He threw me so hard, it was ridiculous. He used me as uke as I am a bit bigger and he wanted to demonstrate how to unbalance a bigger, stronger person. He told me to grab hard, which I did......and then he looked at the class, and said...."Hehe, Stronger fall harder..."......BAM, I was down, over and over again, and the more resistance, the harder I fell.
The point behind that story was, that someone watching with, say an MMA background, might go...."that's ********"....yet, I can tell you unequivocably, that I was not just falling, and I was resisting pretty strongly. He was simply unbalancing me quicky, efficiently, and following w fast techniques that took me down.....