I evaluated one just last month. I replaced the Celestron diagonal with a Tele Vue 2" Everbrite mirror diagonal and an Ethos 13mm eyepiece.
The optics are low quality. It came with collimation about as good as I could get it myself but I wasn't satisfied with the amount of angular distortion and coma.
The Ethos 13 gives 103x but the same field of view as a 25mm plossl. This one eyepiece will do everything the scope is capable of all at once. I tried a shorter focal length but the aperture and optical quality just don't give any more.
The image was much superior to a 80mm spotter in the same price range as the C5 with the standard Celestron parts. For the price, you can only get refractors with very bad chromatic abberation which bothers me a lot. Compared to a Alpen 788 for example, the C5 is far better optically.
Where the C5 falls short is due to the large central obstruction from the secondary mirror, scatter across the schmidt corrector, and a low quality mirror (compared to something like a Questar). The standard prism is also low quality. The C5 I tried had the Starbright XLT coating, but I feel that throughput overall was low even with the TV diagonal. The large CO results in an area of open aperture equivalent to about 100mm, but I suspect scatter and the relatively low reflectivity of the objective mirror reduces the throughput to less than a Leica or Swarovski 80. The CO also reduces contrast and I found the instrument's resolution unimpressive. In theory, the aperture should give 1 arcsecond resolution, but that's just on paper.
Also, you're really limited to 50x-style fields of view because the image circle coming out of the back of the cassegrain configuration is fairly small and the focal length is something like 1350mm. The 1250 is in prime-focus with no diagonal. I had a 2" mirror on this, but I don't think you can use a field stop over about 22mm with it. It will probably vignette.
Another problem with the C5 is the large diameter of the OTA. It won't balance on a photo tripod at all, not without some kind of large counterweight. It needs a single or double-sided fork mount. I tried it with a Manfrotto 410 geared head but didn't like it. A Vixen Porta Mount II would be better. You really want to mount it so the altitude axis goes through the scope's center of gravity.
Do check one out. I don't mean to discourage you because for the price I don't know of something much better. For more money you can get better optics. I would check out Orion Optics UK for a 140mm Mak-Cass, Tele Vue, or one of the alpha spotters.