Read through the Mugen threads on this forum (and other devices' accessories forums).
I think the feedback is generally positive.
After 1 year, my OEM battery was OK, but far from its original capacity (subjective, but still). So any "good" phone battery will degrade over time, it's a very punishing environment.
I don't know how the Mugen will be after 1 year - I hope it's OK, since I bought one 2 months ago. The 1500mAh seems real, and it's started well for me.
I think the Mugen is actually cheaper than a replacement HTC battery (don't even think about the ebay rip-offs), so since they are well constructed I think it makes sense to buy one of theirs.
Also consider a battery pack, you can get a good (NewTrent) 5000mAh pack for 25 or so off Amazon. They are even made with a 11000mAh capacity...