The Celestron C5 is one of my all-time favorite telescopes. The C5 is a great grab-and-go telescope, an excellent travel telescope, and can serve as a complementary scope in other situations. With 5" of aperture in a package the size of a coffee can, the C5 gathers enough light to bring in a wide variety of night sky objects, yet can easily fit in a backpack.
Other telescopes in the same aperture range can fit in a backpack as well, such as the Stellarvue 110ED refractor (which breaks down into several smaller pieces). That telescope, no longer made but available used, will also provide sharper views and allow for wider field viewing, but at a much higher price. Additionally, the Stellarvue scope weighs more and requires a sturdier mount.
While the C5 won't perform quite as well as a 4 to 4.5" refractor, it will come close and do so at a fraction of the size. The small size and light weight of the C5, around 6 pounds or so, make it suitable for smaller and lighter tripods, which often are the main limitations when traveling by air.
I traveled to Hawaii with the C5 several years ago with a modified Orion Sky View Pro tripod (shortened to fit in my suitcase) and a Stellarvue M1 mount, similar to their current M1V mount. I also had an extension column to boost the height a bit, but the whole package was easy to carry in one hand. Any scope in Hawaii will provide great views from an elevated dark site there, so it performed wonderfully in that regard, and it was much lighter and easier to transport than the 110ED which I took along on a different trip.
Other Celestron SCTs like their 6" or 8" scopes will outperform the C5 as will most 4-5" refractors, but it is so small and light that it will allow you to get out under the stars frequently. While the C6 is only an inch larger in diameter, it is closer in size to a C8 than a C5 in terms of overall feel. While still easily portable, the C6 is a little too big for most backpacks. There isn't much size difference going from 4" to 5", but there is a bigger jump from 5" to 6". On the other hand, a C6 will show more than a C5, but then a C8 will show more than a C6, and so on. Even with only 5" of aperture, the best view I have ever had of the Andromeda galaxy from my home was with the C5 and a 25mm Plossl eyepiece. Go figure.
The C5 is sold differently than other Celestron SCTs, which has both advantages and disadvantages. The C5 is sold as a spotter scope and comes with a lifetime warranty and case. The C6, C8, and other SCTs come with a two-year warranty generally and lack a storage case. As a result, the C6 is sometimes sold for less than the C5 when purchased new. The C6 will outperform the C5 making the higher price of the C5 harder to swallow. But, the C5 is an excellent portable scope that can bring in beautiful views. It may not fit into your scope lineup if you already have other small telescopes, or if this is to be your only telescope, but if you are looking for a travel scope to complement a larger telescope you have at home, or need a lightweight grab-and-go telescope, take a look at the C5.
There a couple of other downsides to be aware of. The primary one is that since the C5 is an SCT, and has multiple mirrors, those mirrors must be kept in alignment for optimal viewing. Collimating (aligning) the mirrors isn't hard and is accomplished by adjusting three screws on the front of the scope. However, it can be tricky the first time you attempt the process since often only small turns of the screws are needed. Luckily, small scopes like the C5 hold alignment well, and rarely need touch-ups. The other issue is that SCT scopes also are more sensitive to thermal gradients within the scope. So, if the scope is warm, but the outside air cold, air currents can develop within the telescope disrupting the views. This issue can be solved by placing the telescope outdoors 30 minutes before you are ready to use it or starting your observing session by looking at objects that are less sensitive to thermal issues like star clusters and saving looks at the planets until the scope has cooled down.