It's... okay. Having Deckard's gun is awesome, and sure, using 5.56 ammo is cool because it's a versatile and plentiful ammo caliber but there's not much of a reason to buy it if you already have access to 5.56 rifles (and by the time you reach Novac you'll probably already have the service rifle), since it's not a holdout weapon anyway. I guess if you're playing a crit-character.
True that. If you're character is crit-optimized, That Gun is pretty beastly.
You can get it to almost 100% crit chance with a bit of work and keep it reliably at around 70% without temporary buffs.
When it crits, it does 60 damage by default, even more when using match ammo, and it is even affected by the Cowboy perk.
And then you can use AP ammo for it, too.
That Gun hits much harder than the Service Rifle.
Of course, the Service Rifle has the higher ROF, and is the better crowd control tool.
But still, That Gun is quite awesome for a sidearm using common ammo.
And it is rather cheap, for what you're getting. It's just around 1400 caps with a low barter.
And if you feel it is still too much, just wait for Cliff to go to bed and just steal it from his storage room