Chainrings - always replace with like for like??

Outer chainrings are a very infrequent change... you could have it on your bike for years. So, I would go for a Shimano replacement - upgrade if you want - but stick with Shimano. A badly shifting chainring is a pain in the arse - choose wisely.

I'd suggest that upgrading chainrings isn't always a good move. Shifting will not be as smooth if it doesn't have good ramps and pins, and Shimano ones are carefully designed so that the two chainrings work together for best shifting. The only benefit to be had IMHO is if you are going to Q-rings, and whether you'll find that beneficial is a controversial topic.

Also, if you have let your chain get to the point that your chainrings get knackered, you may be in for a variety of problems, depending on how many components you change at once. These include chainsuck, slipping chain and rapid wear on new components.

If your chain is significantly elongated by wear, you may do well to replace both rings, the chain and the cassette in one go.

Upgrade.  Always upgrade.  

Is it worth rotating the chainring to even out the wear? 

Get the bolt spacing right and, on older setups, there is a world of choice.  5 bolt 130 or 110 bolt centre diameter (BCD) is the older Shimano standard (moving to 4 bolt now on just about all their groupsets) 

The cheapest replacements probably won't have the ramps and pins that Shimano use to make shifting faster/easier under load.

I've used cheap and cheerful entirely plain chainrings without really noticing any difference.  Even if it's slightly slower, how often do you change chainring?

It depends on the crank. If you've got a five-arm crank, then Ogi's suggestion largely applies - just match up BCDs with the new chainrings and you're ready to go.

Shimano started changing things up with their asymmetric X-shaped 110 mm BCD cranks though. Those started on Dura-Ace 9000 and worked their way down to Sora R3000 - the only cranks that still have the old five-arm configuration are the non-series ones like the RS500. Campagnolo and FSA have followed suit with their own proprietary crank BCDs. In these cases, finding aftermarket chainrings that will fit becomes a bit of a challenge, and replacing like for like becomes sage advice.

You can use various different makes - as long as your BCD matches (110 or 130 for Shimano). I would personally suggest to try out for example Rotor ones (with or without Q), but if price is a factor...I think original Shimano are possibly the best choice (Rotor is probably pricier). If you want to go even cheaper, FSA or TA (French company) could be the way to go. There are lots of others (Praxis, Race Face etc.).

O