Here is a different perspective: Having a phone (or other device) that runs an older version of Android allows you to make sure any app you write will run on older versions of Android.

(If you decide to get a separate device just for this purpose, I recommend getting one with low specs, as that will allow you to make sure your app isn't more demanding than it needs to be.)

Hi It's Okay. This one is going to be long as well... That's NOT a budget. For me 600$ is more like fortune.

NEVER BUY A CUTTING EDGE SMART PHONE.

as technology keeps changing and may be you will endup with a regret. So go for something which is cheap and popular. More like under 100$.

I will suggest save rest of money in bank. Make it money in bank!!! Not rig is heavy for a gamer!!!
Get yourself a gaming/workstation rig.

I have no idea if the Blu phones can be rooted, but I presume they can be, the rom appears to be quite nearly stock.

the latest Xiaomi Redmi Note 3 has a 4000mah battery, you should be fine with it :)
http://www.gearbest.com/cell-phones/pp_268449.html

If you don't mind saving money for a not so famous brand (in the west) i think you would have no problem to find this and other china phones in Pakistan, they are a huge thing in India so i guess it's quite the same there; Xiaomi is also the most famous china brand, so i guess they care about the neighbours..

i have a Meizu m2 note and I'm super fine with it, i couldn't ask more for a 140€ device

Heh..I didn't think of it like that, but that only handles one side of the equation! What about other apps that may have come out, not by me, that I'd want to try out? Some game or an attempt at running dosbox on my phone to see if I can use some gog game?

In the sense of having a lo-end phone to test out my own stuff on, I was planning on selling my current phone, but it could serve this purpose as well.

$600 is indeed a fortune, but a fortune that is just at the right amount to get something at the higher end of the current generation of smartphones.
And as a "proud" owner of a palm pilot and an Apple Newton (both as gifts, though, didn't spend money on them :D), I can see where you're coming from about not buying experimental technology, but I wouldn't say that the current generation of smartphones are really experimental.

They already have reviews out about what is good in them and what is bad in them for everyone to judge- for example, some here mentioned a Sony Z5, but it seems that it has only got a 10hour battery, and coupled with an incredibly dense camera that I probably don't need (23MP! :O), I can understand it may not be the right one for me.

I'm not sure I'd get anything meaningful for just $100...even my current phone could probably sell for more than that.
And like I said, I'd get myself a nice gaming rig, but in my current situation (no idea where I'd be living in the next couple months, constant moving about anyhow), it doesn't seem like a useful thing to do just yet...

Will check out the Xiaomi Redmi Note 3 in the local market, though..that battery seems insane :D.

Do you want to play high end games on the phone?

The question is really what you need. A high end phone will give you a better camera, higher resolution screen, and of course more power, but if, as you say, what you want is to use it as a phone and for casual gaming, a $200 phone will serve just as well, and in two years time you'll be able to buy another $200 phone which will have a more up-to-date OS than the $600 phone, and in 4 years time you'll buy another $200 phone which will be better than the obsolete $600 phone. So the $600 will serve you much better if you spend it over time. You'll still be losing some high end features, but from what you say I don't think you really need them.

(Frankly, high end games will likely play pretty well on a $200 phone, so you won't be giving anything up even if you do want them.)

I just went with a Microsoft Lumia 640 myself, it's not Android though. It's gotten a lot of good reviews over here, and wins lots of tests. I waited for a good price on it, and bought it at a sale for 2/3 of its launch price. It seems pretty cutting edge for its low price :)

Heh...I dunno. I'd like to have the chance to if I want?
Like with my current phone, randomly I'd get informed (after trying to install) that a specific game didn't work on my phone. Stuff I just wanted to try out to see what it was about (eg. a lot of Humble Android Bundle stuff like Symphony, some of the adventure game ports, etc.). I guess one of the issues with my current phone is that it is an off-brand phone, so there isn't really any support or documentation that would have the game listed as playable on my device.
Amazon and Google Play tell me when my phone wouldn't run something, and that's usually the way I end up checking. I may not really want to play GTA Vice City on my phone, for instance, but I got it through some amazon deal, and I'd definitely like to experience how on earth it is to play such a game on the mobile :D.

And Random_Coffee, you're the 2nd person to suggest the Lumia to me, but I don't think I'd want a Microsoft one :D

For battery life issue: one workaround is to get either a case with a built-in battery, or a small portable top-up charger. Chargers with the same dimensions as a small pad of the standard square type of Post-It notes can be had for $10 or so, easy to slip into a pocket with the phone.

I consider a microSD slot essential---best way to expand storage for less cash (compare prices of 8/16/32 GB models of a single phone, then compare the cost of a microSD card, and you'll see how they want to gouge you). Plus, it's removable, so if your phone dies, you can still access data stored on the card.

Finally, either buy a really good Otterbox-type case or buy a ruggedized phone. You do not want one accident like dropping your phone in a puddle or on a subway escalator to ruin your investment!

Pretty much any phone will do, they are all more or less the same, just different brands. I'm still using my 2 year old Samsung Galaxy S7390 'Fresh': 512MB Ram / Dual Core CPU. Does all the things I need and can handle many stuff, including playing Quake 3 and Return to Castle Wolfenstein. - Bought for 150e (and I see it going for 100e now).

I still don't see the reason why one should spend 600$ on a phone, in fact most people that do end up not using its' full power. Waste of money if you ask me.

Get a good 200-250$ one and you should be good.

Loving my Nexus 6P. Plain vanilla Android, no bloatware, nice specs and performance, reasonable price. And if you want smaller and even cheaper, there's the 5X.

That's usually the issue. I had a THL phone, an Onda tablet, they're decent but only to an extent. I think that brand name phones won't have problems.

It really depends on how much money you feel you can spare. If you spend $600 and have no problem spending another $600 in 3 years on a new phone, then there's no problem with that. If want to save money, then you buy something cheaper, and it will likely be good enough for your needs.