I think it is totally worth it.
I already had a gtx 260, so i bought the glasses and screen for £310. At the same time i bought an ati 5870 for £300.
I have the viewsonic 120hz plugged into the nvidia card. I have my 24wmgx3 plugged into the 5870.
In the last month i have not used the 24 Nec screen for any games. The reason being, the 3d vision is just to good. It some games eg tombraider, wow, gta4, batman, re5.... its is jawdropping. I simply will never play these games again in 2d.
Some games dont work so well, eg fear 2 (the problem is the crosshair, the ingame one is at the wrong depth. You can use the nvidia crosshair, but you cant remove the fear 2 one. So you have to play whilst ignoring the game crosshair). However the game looks so good in 3d, that i will still only play in 3d mode despite the crosshair problem.
Some games dont work well at all, and are better played in 2d mode eg crysis.
I have far more games that work well than dont.
Another plus is the screen itself (i play ut3 insta, team fortress, quake etc, all in 2d as i take them serious and 3d makes them harder). Compared to my nec 24wmgx3 : I lose 2 inches, fall in res from 1920x1200 down to 1680x1050. The colours are a downgrade. This screen has backlight bleed the NEC doesnt. Viewing angles are horrible compared to the NEC too. However for games, the 120hz is the best. I dont care about any of the downgrades that i listed as the smoothness the 120hz brings more than makes up for it. My frag counts instantly rose.
So my point is, (presuming you have a gtx 260 or better) for the same price of a 5870, you will gain a screen far better suited for gaming than any 60hz screen. You will also gain a far better graphical upgrade than a gfx card upgrade. The 3d looks higher resolution than it actually is, so i dont miss the drop in res. The graphics look way better cos they are in 3d. So to me from my experience it is the best upgrade that a high end gamer could make.
The way i see it is, going from 260- 5870 i would only see a small benefit in a few games. With the 3d a see a HUGE improvement in a lot of games.