I actually enjoy OG a little more then the usual SRW...
The mechs are cooler, I love the characters/music and you're not forced to bench 90% of the female pilots. That said, I also love the main games and the feel you get from controlling Gundams, Mazingers and such.
I think it could be considered a spin-off, but it's SRW just like the mainline games.
"It's a tradition unique to JRPG fanbases to discuss the virginity of characters." - SetsunaFSeiiei BBCP HYYYYYPE
People that got to know SRW by OG first ( GBA version ) often find OG series more enjoyable than regular SRW ( English translation plays a big part here ), I think. I have a friend that got into SRW by OG2 ( GBA ) first and then OG1 ( GBA ) then Alpha Gaiden, Z, MX,...After 5 years, now he still think OG2 ( GBA ) is the best SRW game he ever played.
He also said that playing regular SRW game require watching a bunch of robot shows ( Some of them are just too old or not his taste ) to know all the story lines. While with OG game, he can actually follow a coherent story line. I guess some people find it hard to get into a game without knowing the story full well.
In my case, I think both the OG games and regular SRW are awesome but I prefer games that give me much more choice in using/customizing characters ( OG games win at this ).