All I know for sure is that I love my 510 and my blu, and I have absolutely NO desire to buy an ego. Perhaps if someone came up to me and asked me to try it, I would, just to see what it's like, but unless there's actually something that makes it taste better or just overall more satisfying(like more than just "omfg, it has a bigger battery") I won't ever be buying one. The same goes for box mods and all that other ridiculous looking crap. My 510 suits me just fine.
Your thoughts just go to show that we all different and for some people they find no problem with changing out batteries often not to mention they really don't want or like the looks of the box mods or larger battery models. If it works for you that's great.
I was very happy with my original 510 I bought, but for me, when shortly after I purchased it I wanted to add a few extra batteries to my collection when I looked at the price of the regular 510's compared to the 510 megas (280 mah) I decided that the extra $2 per battery would be worth it to have a bit longer battery life than the regular 510 batts. Less charges would mean the batts should last longer and more than cover the extra $2 per battery I was seeing at the time. Only a bit longer in length than the reg 510 batt didn't do anything to drastically change the look. I was very happy I made my minor upgrade and I'm still happy when I use my 510.
Fast forward to 2 months later and constantly having batteries on a charge was a bit of a PIA although certainly doable. I didn't want the expense of purchasing a PCC as those things aren't cheap. I also knew that circumstances had meant that for those first two months I'd almost always been close to a plug to charge my batts whenever I needed but that wouldn't always be the case. I could purchase several more 510 batteries to cover those times or a PCC but for me the question was if that the best use of my funds. That led me to research an Ego battery. If I bought the battery I also needed to purchase a charger for it. I could find one ego batt and charger shipped for about $29.00. The size of the ego still concerned me but not a huge deal since I still had my 510 to use when I wanted. Long story short Libery Flights put their Ego kit up for sale for $39.99 which would give 2 ego batts, 2 attys(which I was going to be needing to buy shortly anyways), wall unit, and USB charger so I bit the bullet and purchased one. No I can't say I like the ego tons better than my 510's but I do like it and also like not having to recharge batts nearly so often and I don't mind the larger size at all. It doesn't get anymore attention when I'm out in public then my 510 does.
Who knows what the actual life of the batts I have will be but the life expectency of the 510 is 200 charges max and the Ego 300 charges max. If the ego needs to be charged less often AND holds its charge longer the added cost per battery is actually a savings over the 510 battery in the long run for me.