definitely a big saver in time

been using it for over 12 yrs now. use the brand of "gas tite" exclusively-used ward flex just a little bit.
I've used Trac Pipe, Ward Flex & Gas Tite CSS (Corrugated Stainless Steel) & while I preferred Trac Pipe, the company switched to Ward Flex for cost reasons..
have gas piped many new houses completely with gas tite. with the exception of the "stub in" from the meter or regulator, which is black iron pipe, of course. the stub in is also where the gas pipe is grounded.
Here in NY's Capital District, we can use CSS in all indoor applications, & even through wall as long as striker [plates are used at sills & steel sleeves are use at all floor penetrations
i have never seen it happen, but i have heard (talk) of lightening strikes and the flexible gas line not being able to withstand the strike, resulting in failure...
The problem lies in the thin wall nature of the product. A lightning strike can cause an arc from the CSS to ground which will burn a hole on the wall of the pipe. I've had holes burn thru the CSS by an improperly installed (ie. homeowner) electrical box.
The dude used a metal box & ran outdoor wire without Romex connectors. He merely stripped the gray plastic sheathing back & figured the Romex connectors weren't required. He then mounted the metal box to a 2x8 floor joist with 2" screws. I was installing Trac Pipe on the opposite side of the joist & due to the lighting of the crawlspace, never saw the screws protruding. As I began to strap the Trac Pipe to the joist, I saw a spark about 10 feet to my left...WTF? I noticed that familiar ozone aroma & jiggled the line until I saw another spark. I got my flashlght & could THEN see the screws. I had burned a 1/4" diameter THRU the Trac Pipe.
Where the wire ran thru the hole in the box, the copper was in direct contact with steel. The voltage carried thru the screws to the point of contact with the Trac Pipe. I was fortunate that it happened BEFORE I turned on the gas flow.
If house current could do that a lightning strike could cause MAJOR problems
also, the capacity of the line is less than black iron pipe- due to size and the corrugation- so you have to make sure the distance for the added length and the btu rating of the appliance (fireplace, in your case) are compatible-the longer the length, the less the carrying capacity of the flex line...
The distances for either gas line are the same. The ID's if CSS & BIP are both nominal (ie 3/8", 1/2" or 3/4"). There may be agitation caused by the corrigations, but the volume is the same & the same flow charts from the national gas code books apply...