I first found out about To Heart when I watched Comic Party, and I thought I'd give it a whirl. Unfortunately I watched To Heart 2 first so I wasn't expecting much when I eventually got around to watching this.
That said, I was pleasantly surprised at how different the atmosphere between the two shows actually is. Where the sequel feels quite forced, To Heart is far more natural in the way it progresses. The stories aren't that different between the two, and indeed the theme (harem highschool romance), is generally quite common amongst many shows (Clannad, Kanon, Lamune, etc). However when you look a little closer you find that To Heart is actually quite unique in it's approach to the romance side of things.
The story doesn't follow any straighforward plot, but adopts a more episodic approach. This detracts from the various possible romantic developments (of which there are few), but it gives the show a very different feel from the norm. To Heart isn't a harem highschool romance by a long way. It get's misnomered as such because people assume that, because it's based on a dating sim, and has a male lead who meets multiple females, it has to be a harem highschool romance.
To Heart (unlike it's sequel), is slice of life. Nothing more, nothing less.
The episodic nature of the show actually works when you look at it from a slice of life perspective, and the fact that there is no "real" romantic development becomes irrelevant at that point. The true qualities of the show can only be admired when you watch it as a slice of life show, and in this way it reminded me of Aria far more than Clannad et al.
At times the artwork in the show is exceedingly good. The show makes use of watercolour stillframes to good effect, and although the animation is a little dated compared to most you see these days, it still retains a certain "realistic" quality about it.
The music in the show is generally quite mellow, again, reflective of the show's slice of life perspective. The OP and ED aren't really anything special though, and the sound effects, although quite well used, are sparse at best.
To Heart doesn't really have any truly memorable characters (except maybe Multi). Fujita Hiroyuki is generally a nice, caring guy, and Kamigishi Akari is a well rounded, kind-hearted girl. The two have been best friends and neighbours since kindergarten, and the show highlights the strong ties between the two of them. The many female "love interests" in the show are mainly just friends. One or two may have a crush on him at one time or another, but for the most part the "romance" is mainly between Hiroyuki and Akari.
The supporting characters are generally okay, but nothing special really. I found some of them amusing in some ways, but I didn't find any of them annoying (I'll explain why in a bit).
I think if you approach this show with the assumption that it is a stereotypical harem romance then you probably won't enjoy it as much as something like Clannad or Kanon. However, if you approach the show from a Slice of Life perspective then it becomes a nice, relaxing way to spend half an hour.
The main appeal of the show is the "nice", relaxed feel it has. It won't appeal to anyone who wants a good romantic story with continuous plot development (watch Love-Com or Bokura Ga Ita for that). It may appeal to fans of Aria though, as To Heart has a similar feel (and both female leads are called Akari). The reason why I didn't find any of the characters annoying was because of the relaxed feel of the show - the characters didn't bother me in the slightest - which is very much the same feeling I had when I watched Aria.
To Heart is a show that is worth watching if you're in the mood for something nice. It doesn't bludgeon you over the head with drama or romance or thrills of any sort.
It's just.... nice.
