It is entirely up to your personal preferences. I always like "if it isn't broke don't fix it"... With that being said though upgrading to mountain lion and iOS 5 or newer will provide icloud integration. I personally like how everything (calendar, contacts, reminders, mail, etc) is synced by icloud. However Mountain Lion has "rearranged" some things that may provide for a bit of a learning curve. Some users believe the new way certain things are set up are less intuitive. Personally I start on mac with 10.7 Lion, and Mountain Lion was quick to follow. So I don't really have an issue with that.
You should probably visit a local apple store and poke around mountain lion, they have iphones and ipads available for icloud demos. Go see, touch and experience it for yourself to make the best decision.
Upgrading should not affect how your devices work with the Mac. It you were using new devices then it would (it would help).
Considerations when upgrading:
1) Will you need to upgrade hardware too? Newer systems need more RAM. Check system requirements.
2) You may be using older software. Check your versions. Lion and Mountain Lion stopped support for PPC generation software. You may spend $20 on a new system and then $500 on new software (which you might eventually need to update but it is nice to pace it).
3) I can't provide specifics but even if a computer can run a newer OS, it doesn't mean it makes sense unless you need it (and maybe you don't yet). I can run Leopard on my computer but it would not run as well as it does with Tiger and I haven't needed it yet. Personally with your machine I would go for Snow Leopard. It is still available and cheap (not the case with all old systems). It shouldn't kill any software you currently use. It's a requirement for later system upgrades.
4) The one incentive for upgrading further is Apple unpredictability. Post-10.6 all OS upgrades have changed to online downloads. Frankly, Apple could, if they so decided, pull Lion from its virtual shelves tomorrow and refuse to sell it any more. If it did that then you'd be in a mess because except for rare USB drives it was not sold on physical media and Apple pulling it really would stop you from ever getting a copy. No discs means no used installers floating around the used market, and people can't sell you their download rights without giving you their whole AppleID along with all their iTunes purchases, etc. This would spell an early demise for your hardware because you'd be locked into whatever ancient OS you could find on disc and get left behind with newer software demanding newer systems you could no longer buy.
Kappy 08/2012 post on upgrading to Snow Leopard, then Lion or Mountain Lion - https://discussions.apple.com/message/19401628 - including how to get Snow Leopard and Lion since Apple removed them from the online store.