From personal experience, I hate Youtube videos with intros and seeing one, especially a long, noisy, flashy one, immediately predisposes me to dislike the video and the channel even without having seen any content. I also compulsively try to skip intros, which leads to further annoyance as I try to guess by trial and error exactly where the intro ends and content begins.
One thing I really love on Youtube is when people mark the start point of the video on the seek bar, so I can skip to the video itself. That makes a video with intro almost as good as a video without one.
Outros never bother me because I'm practically never in a position where it's difficult to skip an outro: It's obvious where an outro begins, and if I don't want to watch it, I can just close the video, since there isn't supposed to be any video after the outro anyway. I should say that I don't watch most outros either- they are often annoying.
Regrettably (at least for me), it may make more sense to use intros anyway. Sure, you'll annoy some viewers, but many people are too lazy to skip a few seconds of intro even if they'd rather not watch it (ie they'd "skip" the same segment if it was an outro), so you will get away with forcing at least some people to watch your intro who otherwise might not have. It may even make up for the people annoyed by it.
I'd like to point out some examples of good intros/outros:
- Zero Punctuation video intros quite harmless visually (at least for me), but the early videos used to have an intro with music related to the videos topic. This actually made the video more enjoyable and added to the experience - as I was looking at the intro during this time, I imagine it accomplished the branding task quite well. Granted, you could argue that Zero Punctuation was just too good and stylistically distinctive to need any branding... Anyway, at some point, Yahtzee decided to use the same generic clip for each video, apparently due to copyright concerns. Soon after I began to habitually skip the intros - the song wasn't anything special the first time, and anything gets old after hearing it so many times. I dislike the intro and I'm not alone in this - and the intro (even though I skip it) would make me less tolerant to drops in quality (I can easily imagine a situation where I decide not to watch them anymore, but would have stuck with it if the intro was less annoying).
- Again going to Zero Punctuation, Yahtzee adds small jokes in the credits after each video. This is the primary reason why I stick around to watch the credits. On the other hand, the awful intro song comes back, so I usually mute the video when credits start.
- Half in the Bag has a very quick intro, with no music and little sound except Plinkett's voice hoarsely reading the title. Once again, I think overall, this adds to the video and if given the choice, I'd prefer to watch HitB videos with the intros in them. The key here, is, again: Not loud, not too flashy, not too busy-looking, quick, well-made (presents interesting visuals and sound, not wholly divorced from subject matter).
- The Duck Song has a nice outro. It effectively and un-obnoxiously gets the message across. Moreover, given the video, I probably wouldn't mind if the music went on playing instead of fading down. Incidentally, the intro avoids being irritating by not being loud or garish or too long, but it is also unremarkable and if given the choice I'd prefer if it was omitted entirely, as it adds very little despite an unnecessary 2-second silent delay.
- The SMBC-Theater has a short intro, which is graphically fine, but the sound is somewhat loud. The outro is similar. Still, it manages to end just a moment before I begin to feel irritated, so in the end they get away with it. An example is in this video I chose at random.
- Incidentally, this recent SMBC video has a nice, custom intro sequence, which I like a lot.
So as a summary, if you want to have an intro (though this applies to outros as well):
- Not too loud.
- Not too long.
- Don't try too hard to grab attention.
- If possible, a custom intro for each video is a great thing to have.