I have both v4.5 and v5, for the most part i found V5 to be a good upgrade. There are some improvements in performance in V5 in there memory management and use of the GPU that made a small but noticeable improvement especially in reducing stuttering. On the negative side it also made the VC's in some aircraft noticeably darker.
For FSX native aircraft I have never had any problem using it in V5 if it worked in V4 and I use quite a few of them.
For FSX native scenery I don't use very much so can give as strong of a recommendation, P3D5 did update the base scenery library and airport database so there may be some incompatibilities caused like that but if you use lots of Orbx scenery its not that different.
In terms of ORBX, I have quite a large ORBX library and the vast majority of V4 products are available in V5 and you don't have to pay anything extra to use it. There are a few V4 products they did not upgrade, With the exception of CityScene Honolulu which is only available for V4 and V5, the CityScene products are not compatible with V5, I remember that especially as I purchased CityScene Charlestown and Orlando a few weeks before V5 was released and felt cheated that they did not upgrade them.
There are some payware v4 aircraft that I own that are not compatible with V5 and you can normally tell just by looking at the product info page for your favorite aircraft and see if they updated it. Sometimes they work sometimes they don't but I figure if they don't I can always go back to V4 if I really want to fly that aircraft. Again I have never had to pay extra for the V5 version if I had V4.
I upgraded right when it was released and don't regret it, partly because I want to keep the product alive. I also buy X-Plane when new versions are released to keep them in business and I keep hoping they will release a version I would like to use (on that later part I keep being disappointed).
I do have MSFS and use it but use P3D5 more, maybe as more AC are released for it and I become more familiar I switch fully over but there are way to may of my favorite aircraft where there is no equivalent in MSFS.