When you're committed to keeping your car looking nice, the best thing you can do is maintain. If you wait until the car looks filthy, it's a much bigger job.
For me, that usually means keeping up with wiping down every time I've got a few minutes while I'm sitting in the car, waiting for the kids at pick up. That's easy enough, and keeps the dust and childhood stickiness under control. We empty the trash every time we stop somewhere with a garbage can. But when it comes to vacuuming, I generally stick to a once a week schedule, and I go to the car wash with the self service vacuums. At $1-$1.50 a vacuum, it seems like a bargain. Sometimes it takes me two vacuums to get it done, but it still seems very affordable.
However, someone recently suggested that keeping a vacuum that can plug into the car outlet is a better investment. Most are between $30 and $50, which means that they would pay for themselves in less than a year.
The one thing I love about the vacuums at the car wash is the power of the vacuum. When I did a side by side comparison, the portable vacuum did hold up for every day messes. Most decent ones are able to do wet and dry mess.
Is it as easy to use? Definitely. The ones with the best reviews require them to be plugged in to the car's outlet, rather than have a chargeable battery, but the cord is plenty long enough where it isn't a problem. Emptying and cleaning the filter is simple as well.
And as for convenience, it can't be beat. Although it's not difficult to make it to the car wash once a week, having something that can literally be stored in the car and used at any time I have a minute or two is pretty amazing.
When you want to maintain a clean car, the benefits of buying a vacuum make it worth the investment!