I own both the LG 25UM58-P and LG 29UM68-P and these are my thoughts:

Pros for both monitors (and ultrawides in general):
1. For gaming, ultra wides are awesome. The extra field of view is amazing. It definitely adds an advantage - even if it's just peripheral vision, it all helps.
2. For productivity, ultrawides are definitely a nice way to get my resolution on the cheap without getting much performance impact on your PC (unless you can afford to go for the 4k x 2k ultrawides).
3. IPS panels look beautiful.
4. It absolutely is easier to multitask on an ultrawide than two 16:9 monitors - although some applications don't scale well so this isn't always the case.

Cons for both monitors (and ultrawides in general):
1. Ultrawides are typically shorter in height than their 16:9 counterparts. This is because of the 21:9 aspect ratio stretching the screen without the need for it to be physically as high. This means the pictures can be a bit misleading sometimes.
2. At a certain point, the screens are so big you actually have to move your head to see things in the outer edges of the screens which is a bit frustrating if you're using tools, like anything part of the Google Docs framework, where the buttons are situated in the outer edges of the screen.
3. Some games don't support 21:9 resolution, or if they do, they place UI elements on the far edges of the screen which makes it difficult to absorb all the information you need to absorb without having to move your eyes around the screen a lot.

Thoughts on the LG 25UM58-P:
1. Physical dimensions make it 25" horizontal, meaning it's vertical height is more similar to a 20" 16:9 screen than it is similar to a 23" 16:9 screen (so in multiple monitor situations it can look a bit funny).
2. The lack of FreeSync support makes it lower value-for-money than the LG 29UM68-P for gaming, so if you can afford the extra ~R1500 it's worth the difference.
3. To me this monitor strikes me more as workplace monitor than a gaming monitor as it's not too large for a desk with multiple monitors but comes with the benefit of extra horizontal resolution.

Thoughts on the LG 29UM68-P:
1. Because it's a 29", 21:9 monitor, it's physical height is about the same of a 23" or 24" 16:9 monitor.
2. The AMD FreeSync technology is really awesome,.
3. The screen is awesome for gaming (most of the time), but not as awesome for productivity. The physical size of the screen makes it extremely difficult to use comfortably without having to move your head around to look at everything on the screen. If I had to be honest, I wish that 29" monitors were only curved. A flat panel this large is uncomfortably big.

Final verdict:
If you really want an ultrawide and you want it for gaming, the best one to buy would probably be the LG 29UM68-P, but I think a 2k monitor with 16:9 ratio is probably better (and not a whole lot more expensive). If you want an ultrawide to improve productivity on a budget, the LG 25UM58-P is the best value for money.

For both monitors, however, I do not regret buying either.

Loving this new monitor! The colours are great, movies fill the screen, and the software makes changing settings easy. This is not a gaming screen, the size is not ideal for games, but as a video editor using this with Premiere Pro is great. Really nice screen and can't be beaten at this price point. :-)

Hole new way of looking at your games. Did not think it would look so good. For the price its worth it.