I like them but I prefer supporting them with hoops. As celery and onions can tolerate light frost, you probably don't need the covers for 27 degrees, but they can't hurt of course.
The colder it is, the bigger the difference in temperature. On the coldest night of this winter, it was -35 F outside and +8 F underneath.
I took some temp. measurements last fall; see below. These row covers are in my unheated greenhouse which retains no nighttime heat on its own.

Month ambient high ambient low; RC high RC low
Oct 56 32 ; 72 40
Nov 28 14; 48 26

Today 10 -9; 44 19

I haven't tried one yet, but on my drive home 3 days a week, I see a bunch of houses that have row covers on their lawn so they can get early crops! I also see different houses with greenhouses, some with 5-10 of them on their properties!

I think I might try one this year, not sure yet though.

I use them for insect control, and they are very effective. Row covers keep SVB & squash bugs from attacking my squash seedlings, and prevent early attacks by cucumber beetle - and the resultant wilt - on my cukes. I cover my peppers with them for saving pure seed. Never tried them for Spring planting, the cover I use has almost no heat retention.

Appreciate the replies.

Was supposed to be 45 degrees here today - but it still is only 30 degrees! I did read that celery & onions tolerate light frost - but I would think a "light frost" would be anything between 30 - 32 degrees. A "hard frost" or freeze.. I'd think that would be anything below 30 degrees.

Weatherman says it is to warm up all this week.. we'll see. Just would hate to lost these onion & celery plants. I do have plenty still under the grow lights downstairs, but they are not nearly as far along as the ones just planted.

Onion seedlings tolerate a lot more frost than celery, IMHO. But I am puzzled at what zeedman says, that he protects squash seedlings with the covers. I am in MI, and by the time the SVB descend on my plants, it is late June and the plants are two feet tall (and they are really difficult to cover and seal, with RC or with tulle). What am I missing?

Well, I'm trying one for the first time. Put out my broccoli transplants yesterday, since I've run out of room inside. At first I was worried that they were going to be beat to death by the wind rippling the fabric, and was starting to regret putting it over them. But today it is relatively still and the plants are fine... and I saw the first cabbage moth of the season and thought "HA! You're not going to lay your eggs on MY broccoli!!!" I'm hoping the minor bit of frost protection the RCs provide will get them through any borderline cold nights we may have from now on...

I would love too, but can only get to my garden 3 days a week. Im afraid they will cook if not there daily to open and close to prevent overheating.

i use them to keep the flea beetles off of the eggplant and the squash bugs (one of the banes of my existence) off the squashes... not so much for temperature control... they definitely help with the bugs.