I never did get to review the Z90, so I can only compare the Z25 to one of its other predecessors, the August 2008 EX-Z85. the main difference between the two cameras is the three-megapixel increase in sensor resolution, and unfortunately this has had a negative effect on the camera's performance. The Z85 was a very quick little camera, but the extra data processing for that bigger image size has really slowed things down for the new camera.
The Z25 starts up in approximately 2.5 seconds, which isn't too bad, but in single-shot mode its shot-to-shot time is now 3.7 seconds, which is disappointingly slow. Even in continuous shooting mode it still takes 3.1 seconds between shots. There is a high-speed continuous mode which shoots at approximately 3fps, but is can only take eight shots and is limited to a resolution of two megapixel.
The annoying thing is that the autofocus system is one of the fastest I've ever seen on a compact camera, quick even by Casio's usual high standard, and the single-shot shutter lag is almost non-existent. I'm fairly sure that with a lower-resolution sensor the Z25 would have much better performance.
Image quality is also a bit disappointing, and partly for the same reason. Dynamic range is extremely limited, especially at the higher end of the range, with badly clipped highlights and charge-leakage fringes obtruding into shadow areas. The lens is also less than ideal, with significant chromatic aberration towards the corner of the frame. I also found that some shots had uneven focusing, with one side of the image slightly blurred, although this may have been a problem with my review sample because I've not encountered this with previous Z-series models using the same lens.
One pleasant surprise is image noise control, which is very good, producing nice clean images at up to 400 ISO, and usable shots even at 1600 ISO, with good colour rendition and exposure. Low light performance is also much better than expected, and the AF system copes in very low light despite the lack of an AF assist lamp.
The Casio Exilim EX-Z25 is an ideal camera for casual social photography, small and light enough to take anywhere. Build quality and design are first rate, and it has a good range of features to cope with almost any situation. It is let down though by sluggish performance and slightly disappointing image quality, but for the price it's still a bargain.