I loved the video review, it's perfect! Short, to the point, if only all video reviews could be like that...
I would say that the choice of Plex vs. Air Video HD or StreamToMe has to do with how you store and use your media. Air Video HD and StreamToMe are great for small collections, but past a certain size you start feeling the benefits of Plex's automatic indexing and cataloguing: as long as you follow simple conventions ("movie title (year).mkv" or "Series Title/Season X/Series Title - S0XE0Y - episode title.avi" for instance) it will automatically find movie or show information, artwork, posters, etc... allowing you to search by a ton of criteria such as year, actors, directors, writers, genre... without having to input the information yourself. Priceless.
If you have a small collection on your Mac, AirVideo HD or StreamToMe are all you need, but past a few hundred files Plex really shines. Another huge benefit is that it's available on pretty much every platform under the sun: Mac, Windows, Android, iOS, Linux... even GoogleTV and ChromeTV! It's nice to be able to run one server and give access to all possible devices in your household.
My recommendation for a standalone server: skip the overpriced NAS boxes and instead go for an HP Proliant mini G7, which only costs $200 without disks, and install the fantastic OpenMediaVault distribution on it. I can't say enough good things about this incredible free Linux distribution, which is remarkably easy to install, configure and maintain even for someone who has never touched Linux before. The Proliant has 4 SATA bays and comes with a 250GB drive; it has room for a DVD drive, internal SATA and USB ports and two PCIe slots. I moved the 250GB drive to the motherboard's SATA port and used the other SATA port for a 4TB drive, and filled up all four bays wth 4TB drives, grouping the five 4TB drives into a RAID6 array, giving me 12TB of redundant storage. This box has been running flawlessly for 2 years.