We have a 2015 351DS, delivered in May 2014. Overall we are very happy with the MH - the layout is perfect for us (although I do envy the king size bed) but it works for our current camping lifecycle.
A few things needed repair - the leveler motor crapped out on our 2nd trip - it would fire up and then short, tripping the breakers. Ended up having the unit at the dealer for about a week while they troubleshooted the problem - FR had never seen this issue prior - but they duplicated it and decided the fix was to replace the whole motor - but none were in stock (as it was a new part) so it took about 4 weeks for it to arrive (luckily, one doesn't need levelers always) so it didn't impact our camping to much.
The LED awning lights also died about 2 weeks before the year warranty ran out, so had those replaced as well.
We've done a fair share of modifications too - had the dealer add another fantastic fan with vent in the bathroom, move the tow plug from the bottom of the hitch to the top (people reported it getting knocked off) and had a PI power surge protector hard wired in.
In addition we added a set of speakers above the driver and passenger seats (so you can actually hear it when driving!) replaced both TV's and brackets, built shelves in the cabinet next to the bathroom, and build out the space behind the bedroom TV and put the TV on a door. Plus the usual organizational stuff. I also recently purchased some clip on LED lights, since I can't seem to have enough light when I am trying to read at night. So far, it works great. We also purchased some screen panels to hang on our awning to help shade the area when the sun is shining the wrong way. They are awesome too.
Always something else to modify - that's what I have discovered in owning an RV.