Chansey worth levelling?
Answers
i would save the candies cause u might catch a better blissey in the wild with better stats/cp instead of powering up a chansey. You can however power it up to at least 1k cp and then put it in the gym if you constantly take over gyms. Make sure u also put her as ur buddy no one else is really worth it assuming u already have lots candies from snorlax, rhydon, vap, dragonites etc.
As long as you are happy with its stats and moveset it is fine to spend candy since you can evolve her into Blissey when the time comes but you have to basically accept it might be your only Chancey.
Chansey is actually quite decent to put in gyms but only as a number 10 who will slow things down and therefore will be the first one kicked out as well.
I powered one up to 1200ish and it sort of works.
She's effectively impossible to prestige by more than 100 points per battle (I tried just about everything I could) and requires such a high cp Poke to kill her that you can't really prestige the next defender or two above her. Putting her in a gym does effectively lock the gym at that level.
Whilst she's a pain to battle and at 1200 cp is probably impossible to kill by casual players at lv20 or below, she can be killed by a single high level Dragonite/Rhydon etc. which most serious players will have, and her low cp will mean you're probably going to be out of the gym pretty quickly.
In practice I've found her a lot less useful than her tankiness suggests.