Yes Platinum does get you room upgrades, CP Lounge Access, free internet occaisionally, well worth having in UK where I book most of my non-IC nights.

You need to know the market conditions in the country/city where you will be staying. Seems to be a wide variation in treatment worldwide although most negative plat upgrade statements , from memory, may relate to USA.

If you use points for an IC, expect higher 'in hotel' costs than you get at your usual IHG HI hotels so budget for that. IC's are rarely going to upgrade a Plat, so I would stick to HI/HIX or CP hotels, If CP and HI both want 25K points I would choose the CP. HI could be as low as 15K.

As always if you do not want to end up with a a basic room, as on points bookings, I advise you book with cash the room type you want.

I stay in CP/HI/HIX a lot in the UK, often London, but also right across the country eg Portsmouth/Leeds/Manchester/Birmingham/Stoke/Glasgow/Edinburgh and other cities, and in the last few years, I am 100% for upgrades, 150 nights plus. (and without having to ask before arrival or during checkin ).
Also Germany seems a good country, Berlin CP, HI and Frankfurt HI all upgraded me on stays.

Some were Paid, some 2-4-1, some prepaid-A, some points, even the odd points breaks, all my bookings upgraded. I even got a suite at HI Oxford Street twice on weekend stays and that hotel only has 4 suites.

In London both HI's and HIX's can be, and are 25K, and in London HI's can sometimes cost a fair bit more per night than a CP which makes the CP a good cash price as CP also a better stay, with a lounge in UK, unlike UK HI's.