While it depends strictly on the campaign, brand, product, in general including video on a landing page is a great idea, as it is more attractive, grabs user attention much easier and leads him from A to Z through the communicated idea.
You jus need to stick to some good practices about it, especially:
- do not let your video be too long (30-40 seconds will do in most cases)
- remember to provide the video in a form that will let users see it also on mobile (thus, prepare also a mobile version of the landing page)
- if your video loads too long, users will not wait - balance the quality against transfer rates & load times to find your optimum. A good idea is to provide some responsiveness here anyway, to serve lighter content on slower connections and small screens (mobile) and (just a litle bit) heavier on fast ones with big screens (laptops, and mainly desktops)
- still, you can provide some alternative content for devices that won't be able to display the video content
- as John said, you shouldn't hit the user with autoplay (or he's going to hate the brand). Users can be viewing your microsite at night, with children sleeping next room for example, so this is a very bad idea. Video should be played on demand.
- HOWEVER, you can build some nice mechanism playing a short, silent, looped teaser video to attract user to view the further video content. This is the idea behind some of YT advertising forms. Consider this, as it can be quite attractive for the user and make him continue to the main video.
By the way, Youtube is a great example to follow - I mean their ad forms. Read more here: http://support.google.com/youtube/bin/static.py?hl=en&topic=30072&guide=30071&page=guide.cs&answer=187015, especially the Load requirements and Video/Animation sections.