I have the BlizzardPPC+BVision combination. They were expensive enough when they were new and they've only gotten moreso since.
My first caution then is the price. I really don't believe the classic PPC cards are worth the crazy prices they fetch. However, if you insist...
As for an objective opinion on performance. Well, I have the 040 version. Under OS3.1-3.9, I use the PPC principally for datatypes and media playback. There's no denying that even with the dreaded context switch overhead that it kicks the 040's ass for that. However, I reckon a fast 68060 would do just as well in many cases and potentially better in some. Beyond that, I for the various WarpOS / PowerUP applications and games that were originally written for it. Are there enough of them to be worth it? I'm not sure. Probably not, in reality. Summary: an expensive co-processor for the OS without a large enough library of software to really justify the price IMO.
Then there's OS4. At least your PPC is getting some use, but while using AGA only is possible, but you'll have to make all the same basic cutbacks you'd have to do under 3.x to get usable performance. Chip RAM access is as slow as it ever was and as the PPC can do much more work per unit time than the 040, those wasted cycles spent reading/writing ChipRAM equate to a bigger percentage reduction in real performance. It doesn't help that out of the box, it tries to use display settings that are just too slow (256 colours, gradients, solid window moving/sizing etc).
In short, unless you intend to use a graphics card with it, forget it. With a graphics card, even the BVision, there are some surprises. I found that a few 68K applications I regularly use on 3.9 under RTG ran considerably faster on 4.1 with RTG. I'd say that basically, 4.1 classic is for users that already have PPC and associated expansions rather than someone looking to upgrade.
If you must go PPC, then I'd suggest you focus on getting either a new Sam on which to run OS4.1 or a PPC Mac to run MorphOS (note to self, one has to be careful mentioning these in the same paragraph as it can be rather like using a lighter to illuminate a warehouse full of explosives during a blackout). For some, the choice between the two has become akin to a religious divide but objectively, the latter is cheaper and has been in development for longer. Be prepared to be flamed either way, if you go for 4.1 you'll be regarded as insane by some, if you go for MorphOS you'll be branded a heretic by others

Personally, I'd suggest that if you want to stay "classic", look for the fastest 68060 card you can get and give PPC a miss. Otherwise you'll need to buy more than just an already priced-as-if-made-of-gold PPC board for your A1200.