Having recently purchased a 2351D with liquid springs I thought I would offer my opinion on them. After looking over the PC floor-plans, my wife and I decided the 2351D fit our usage the best. As luck would have it, PC happened to have two 2351D units in stock. One with LS and one without. Having the opportunity to drive two identical floor-plans with and without liquid springs back to back, it was a very easy decision for me. In fact, after driving the unit with LS and then switching to the unit without LS, before we had reached the end of the side road that PC is located on (which isn't rough at all) I told Bob White, we can turn around. I was instantly sold - even with the substantial additional cost. It was a night and day difference. Without the LS I could feel every crack in the road. The best way I can describe it is; with the LS you still feel the road imperfections, but it's a much softer, mild feel. Without the LS, and considering that these are built on a heavy-duty box truck frame, it's a hard, jarring feel to every road imperfection.
To me the additional cost can be justified in several ways. 1) A much more comfortable and quieter ride. 2) Much better stability. They make a noticeable difference in turns and lane changes. 3) Much less hard-jarring of the living section which I believe will result in fewer long-term issues (Remember that the entire time you're on the road it's like putting your home through a moderate earthquake. On some roads it's more like a severe earthquake.) 4) It's nice to be able to adjust the ride of the coach to fit the situation. 5) Part of the cost well be recouped with a higher resell value.
In fact I am so sold on them that if it was a budget issue I would forego other options to be able to get the liquid springs. If anyone is on the fence about them, I hope this helps.