The bottom line first: I'm very happy with this device. I own and still use in different settings the G2, B2, and G3n. (I do not like the G1 series.) This is clearly the best, and the others are very useful.
The amp models, a weak point of the MS50G, are so much more articulate -- in particular in gain control -- that the + is like an entirely different device. Surprisingly, the Fender models are an exception (too much overdrive for me); I like them better on the 50G, and I also like the Bluesbreaker on the 50G. The Marshall and Vox models on the + are far superior and cover all the territory I occupy. The HiWatt is still fine. The high-gain models are a nice suprise for me. I don't play metal, so can't advise you there, but for rock and slide, the Krampus, Diezel, Mesa and Rectifier models are much more interesting than I expected.
The reverbs and delays are less muddy than the 50G. The choruses are fine; the tremolos sound weak to me. The compression is much better than the 50G; that annoying "pop" is inaudible for me.
The drives on the + remain less articulate than their models, and when combined with the amp models, sound harsh to me; I would advise putting a separate drive in front of the + if you want to add another gain stage for solos. If I find a solution on the + I'll advise.
I had a disappointment when testing: I bought this device (at a knockdown price of 121 euros new) for the polyphonic octaver, hoping it would offer the range control of a Boss OC3. It doesn't. That's an issue for me because I play in a guitar and percussion trio, and the Boss allows me to shift only the low strings, creating the impression of a bass guitar. That said, the octaver on the + tracks better and sounds much better than the one on the 50G. I expect I'll place the + between the OC3 and a 50G for time-based effects and noise suppression.
The interface on the + is very different from the 50G, and though efficient, it is less effective for live use, because there are more steps required to change effects in the chain. (By the way, there was NO manual in the box. I figured out the device on my own.) In other words, it takes longer to change, say, from a tremolo to a chorus in the same slot. The obvious solution is to arrange patches that embed such variations in a row, because shifting from one patch to another is indeed faster on the +.
As I write, I just finished tweaking a few patches for specific songs, and every time I nailed one, I thought, "Sounds great." If you are on a budget and own the MS50G, I'd say you're already well equipped. If you have the money for an upgrade from the 50G's amp sounds, there it is. Stop smoking a pack a day and you'll have the cash in ten days. I'd rather make my money sing than set fire to it. The MS50G+ isn't a Helix, but it's one-fifth the cost of the cheapest Helix, and it does what I need to get done. Firm recommendation.