I love Enterprise, so finding a fan film set in that era was a surprise, and I ended up enjoying this more than I expected to. Most fan films are across the board awful, but Horizon actually had more good than bad going on.
The ship and battle effects were pretty great, and other than the pimp collars on the SF uniforms, the costumes were quite good. The Iconians alien of the week look was pretty crap and the floppy holster in the opening scene didn't exactly inspire confidence, but things improved once past the prologue. The visual effects to disguise the CGI sets was a good choice, considering the low-low budget, and I was actually fooled at times into thinking the production had somehow gotten ahold of discarded Enterprise sets.
The story was OK. Not thrilled with yet another world-destroying super weapon, but I always liked the TCW plots from the series, so seeing more of Future Guy and his machinations here was cool. Nitpicky - Not having the Enterprise lead the assault didn't really make any sense in-universe, since Archer has fought off how many alien threats to Earth by that point, including the Romulans? Having the Discovery support the assault and simply be the one that conducts the attack run on the weapon's aperture, leading to getting sucked down the time tunnel, would have worked just as well.
The side plots with the Romulan infiltrator/Chief Engineer and Captain/Helm Officer relationships kind of fell flat, because they were underdeveloped. Too much was told about how various people felt about things instead of having scenes spaced throughout that effectively showed those interactions. The captain's dreams could have been an avenue to show his relationship with the helm officer in flashback before they morphed into the causality warping visions, for instance. A few more passes with the script to punch up the dialogue all around would have done wonders. I could see where they were going with the infiltrator/engineer stuff but the dialogue just wasn't there.
Similarly, I liked the decision to go with a more stoic/sardonic older captain, and in some scenes it worked really well. I even laughed at a few lines (that were meant to be funny). In the first battle scene and a few of the longer exposition scenes, though, the actor wasn't given much to work with on the page.
Acting overall was a bit of a crap shoot. The Captain was good (after that first battle scene), though his physicality during the shootouts was weak. Daikon was a decent villain with a clear, distinctive voice and great screen presence. The helm officer was good, too, especially during the third act when she wasn't just a dreamland burn victim. Was it T'mar, the infiltrator? She was good but could have been much better if she'd had better lines to work with. The Ensign Sato stand-in and the engineer were meh.
The tactical and science officers were, unfortunately, pretty horrendous. Any time I got lost in just enjoying the show, I could always count on one or the other to wobble into the scene and drag everything down into a charisma black hole. I've heard more evocative line readings from Charlie Brown's teacher and parents.
"Deck whatever is damaged, Captain." *blank, expressionless face, unmoved by fires and explosions right behind him*
"I'm getting some sort of readings or something about some stuff, but it's like weird. I don't know..." *voice trails off into mumbled nonsense as his soulless, staring doll's eyes sap your will to live*
Seriously though, for a fan film with a bare bones budget, it's pretty amazing what these people pulled off. I couldn't even get through the one with Walter Koenig and Captain Cameron after two tries and those guys are supposedly really reals actors, but I watched and enjoyed all of Horizon in one sitting. Best Star Trek fan film I've seen by at least a parsec.