Aside from the obvious benefit of being able to tell your friends that you own a strap-on, they are the bottom of the barrel in the hierarchy of “modes of vehicular bike transport.” They’re far cheaper than roof or hitch racks (roughly $50-150), but they generally don’t offer many (if any) “features.” It generally takes a fair amount of finagling and padding to get bikes mounted in such a way that they’ll escape the drive unscathed. The rack itself or the bikes hung on it (particularly the pedals) can also scuff or mar the paint on your vehicle, so additional padding may be required there. If your car comes with a sweet spoiler, it’s most likely going to be in the way of the strap-on rack. All of that padding and adjusting can be pretty time consuming, so they score low on the ease-of-use metric.
With some exceptions, strap-on racks also tend to cut off access to the back of your vehicle (or at least make it much less convenient). Furthermore, sometimes strap-on racks will leave wheels dangling directly in front of the car’s exhaust pipe, which is a great way to melt your tire.
As with hitch racks, bikes on strap-on racks can be tricky to lock. You’re left with running a cable through them, which is probably going to involve you climbing under the car to find something to loop the cable through. Finally, odd frame shapes and small kids’ bikes can be difficult to mount on the typical strap-on rack, which has two parallel arms upon which the frame rests.
With all that said, aside from being way cheaper than the other options, a basic strap-on rack can be made to fit most vehicles. Also, because they’re fairly simple, there’s not much to go wrong with them. A bottom-of-the-line strap-on rack that I previously owned lasted for years and accompanied me through many thousands of miles and a number of different cars. Nobody ever approached me at the trailhead to ask me how I liked it, but for 50 bucks, it got the job done.
Another Option: Buy A Pickup Truck.
Pickup trucks are made to carry things. They’ll carry your bike, skis, kayaks, dogs, and friends (all at the same time if you want), and if you need to carry even more crap, you can still put a hitch and/or roof rack on one. They’ll also carry manly things such as dirt, lumber, large rocks, and other heavy objects as depicted in any given Ford/Chevy/Dodge commercial. If you need further insight into buying a pickup truck, there’s plenty of info out there.
A few companies make bike-carrying accessories that are specifically designed for pickup trucks. The most common of these is the tailgate pad, which allows bikes to simply be slung over the tailgate without scratching up the paint (on the truck or the bikes). While a full-size pickup truck will easily accommodate six or more bikes over the tailgate (fewer if you have a topper), that system doesn’t work as well with cross-country or road bikes (or kids’ bikes). Even with the padding, it’s also not a great idea with bikes that have thin-walled downtubes that may be prone to denting. Those issues aside, it is hands down the easiest way to carry a bunch of mountain bikes.
At the risk of stating the obvious, before you go throwing money at your local rack vendor, you should be considering what you need to carry. As mentioned earlier, it’s worth considering not only your own gear, but also those that may frequently accompany you on outdoor excursions.
All in all, if you’re only looking to carry bikes, hitch racks are an attractive option because of their ease of use and the fact that they can be removed from the car quickly and easily. If you need to carry things other than bikes, roof racks are generally going to be your best bet. If, however, you’re on a tight budget, strap-on racks will get the job done for a very modest amount of money.