The reactions run from one extreme to another. While some just aren't quite sure what to make of the phenomenon, others have made up their minds pro or con.
Fez developer Phil Fish recently spoke out against YouTubers on Twitter, angered that they "freely distribute" games on YouTube and make money from it. This stance makes some sense; with single-player, puzzle- or narrative-based games, YouTube exposure has the possibility of severely harming their commercial viability. By now, you may have heard someone say "I'll just check it out on YouTube."
Nintendo, for its part, seems to feel similarly to Fish -- that YouTubers are profiting from its hard work. The company had YouTube shut down videos of its games or claimed their ad revenue; it's now in the process of drafting an affiliate program that would allow YouTubers to split ad revenue with the company.
And right now, Kerbal Space Program is offering to share revenue with YouTubers when viewers purchase the game via official affiliate links placed in their videos.
Rather than a punitive "take it away, and then give some back" measure ala Nintendo, this is perceived as a good thing by both developers Squad and the YouTubers. By doing this, the developers incentivize people to put easy "buy" links in their videos that they might otherwise omit; meanwhile, the practice opens up a new revenue stream for the video producers.
Companies involved in this space sometimes act as networks, sometimes act as brokers for ad deals, and other times help develop talent or work to increase their audience share. PewDiePie, for example, operates his own channel, but he works with Maker Studios -- which brokers ad deals for him -- rather than going it alone.
There are a number of companies involved in the space right now; some (like Felicia Day's Geek and Sundry and Chris Hardwick's Nerdist) were formed by established talent; others (like Yogscast) grew from their initial success as YouTubers to become networks.
Still others, like Machinima, were formed for entirely different purposes -- in this case, hosting original video content created in game engines by fans -- but eventually went down a different road as it became more lucrative and culturally current.
Maker Studios. A very popular "multi-channel network," or MCN. Home of PewDiePie, and acquired by Disney for as much as $950 million.
Polaris. Another popular MCN, which has worked with TotalBiscuit and PewDiePie.
Yogscast. YouTubers who became a collective, and now produce a number of popular shows.
3BlackDot. A startup co-founded by YouTubers Adam "SeaNanners" Montoya and Tom "Syndicate" Cassell, which produces videos, marketing, and even games.
Geek and Sundry. A production company and network founded by actress Felicia Day, who found success on the internet as creator, writer, and star of web series The Guild.
Nerdist. Started by comedian and TV presenter Chis Hardwick, Nerdist produces content for both the web and television.
Revision3. Another production company/MCN; originally a startup but now owned by Discovery Communications, owners of The Discovery Channel.
Machinima. Originally founded to showcase original videos made in game engines (known as "machinima," hence the name) it has shifted focus towards being a video-game related MCN.
AwesomenessTV. A MCN which seeks to develop new YouTube talent from the ground up.
Fullscreen. Another Hollywood-based MCN that has been targeted by takeover speculation.