This is a very old debate, obviously. Benefits of starting with jhana or using it as a basis are that it is more fun, allows more early feelings of success or that something is happening in some people who are inclined to it, develops concentration more easily as it is more interesting and compelling than the Three Characteristics for most people, and yet it clearly can gum things up, and it is very hard for people to investigate jhanas honestly, as they tend to cling to and stabilize some pleasant factors as a background even if they are trying their best to simply see the Three Characteristics of the sensations that make up those jhanas. Thus, if one already has jhana, getting completely out of it into pure insight practice can be difficult. If one doesn't yet have jhana, then one must weight the pros and cons before beginning. If one is a stream enterer or above, the danger is much less, as there is the ability of naturally see the Three Characteristics and the cycles of insight will keep one moving along, so while jhanic focus can slow things down a little, it can't lead to what I have sometimes seen happen with jhana practice in people who are below stream entry, namely that they get stuck for years to decades and don't progress. Also, some people will get good enough at jhana that they mistake it for an ultimate attainment or a mark of realization, which is relatively common and a tragic error. More to the point, if you have jhana and want to use it as a basis of insight, simply begin to notice each little flickering sensation that makes up the whole experience come and go, with particular attention to deconstructing the key pleasant factors, as they are pernicious tricksters, and it is harder to perceive suffering against a background of peace or bliss. Thus, I say, get stream entry at all costs, and then play with jhana as much as you wish, and it is easier after stream entry, as one can go the other way and convert vipassana jhana to samatha.
