I have been a tenant of the Horizons since 2003. I have lived both with roomates in one of their 2b/2ba townhomes and I currently live alone in one of their 1b/1ba apartments. I have never really had a problem with them. When i moved into my 1 bedroom this summer, it was freshly painted with brand new carpet and all appliances were in very good shape.
I will admit, rent is fairly high (860 for 1 bedroom) and i know they tend to raise it come renewal time. I guess I can't blame them though, since their apartments are in such a high demand (this summer I was 13th on a waiting list to get one of their 1 bedrooms). They must be doing something right. I guess it's a combination of location (you really can't ask for a better location if you want to be close to both Denver and Boulder) and amenities (especially their gym and clubhouse)
I would say parking is adequate since every unit gets their own garage and there is plenty of off the street parking. So unless you have 3 cars, you should be fine. I have never had any problems with guests finding parking either.
Noise is minimal considering it is such a huge complex.
The grounds are well kept and it seems like an overall safe complex.
In my 4 years of living there, I have only had one thing break (the furnace) and it was fixed the day I reported it without any guff.
There is a good mix of college students, young professionals and families living here (and it's like that at all the apartment complexes in the area... you can't avoid it).
From my limited interaction with the staff, they seem reasonable. It might not be the most personal interaction considering the hundreds of units that this place has, but it gets the job done.
The one complaint i would have is the mail service. Since there are something like 200 mailboxes all in one area the mailman often misthrows mail (especially since all the apartments in that subdivision share the same address just different apt #s) but thats not really the fault of the complex but more so the mailman.
Overall I like it and have no significant complaints. It's a little expensive, but if you look at the prices of other complexes around they are all in that range. I would have to say that compared to other complexes in the area the quality of their apartments is above average. I'd live there again.