What is the forums opinion of SVS subs?

In my personal experience, if spending less than $3-4K per sub, SVS takes the cake. I have two SVS SB-4000 in my room, which is 18x23 with sloped ceiling up to 18’, and the room is open to another room towards one back side. It’s a lot of space and these guys are fast and deep. I tried two SB-3000s and they were slower and not enough for the room, but still great considering their price against a pair of Rel T9i I demoed a week prior. The SB-3000 blow those away. 
 

I did also try a REL 212 SE and it is faster than the SB-4000s, but I couldn’t live without the smartphone integration capabilities of the SVS. Phasing is especially important to me, and REL doesn’t offer that until you get up to their reference series which run $4K+. At some point I will likely upgrade to something in that line. 

I own the SVS SB2000 Pro. It sits upon my homemade bungee trapeze which isolates the sub from floor. While waiting for delivery,  I didn't have much hope, To my surprise, the blending with my Klipsch Lasalas is wonderful. I had all but given up on ever getting a sub to play nice with the big floor standers. Previous subs include Velodyne, Polk, Sunfire and other brands. See? I really tried. Honestly I purchased it expecting to return it as a failed attempt or try again by getting up SVS's product line to a bigger better unit. (Crutchfield returns policy is easy)  Once set up, the sub did what every sub should- It simply disappeared and allowed the music to become the focus! Five Stars

I use a SVS PB2000 pro sub with my JBLL100 Classic speakers and the single sub really made my system go from pretty good to incredible.  Like others have said, their software to integrate the sub is excellent and easy to use.  You can make adjustments on the fly too. I was going to add another of the same sub but the system sounds so good with one, that I don’t want to spoil the magic. 

Compare the warranties before you decide.  BTW, I've owned 2 SVS subs and both were problem-free.

the REL is probably the most musical sub on the market, integrates extremely well with your speakers.