Pros: Neutral bass - no booming. Nice shape, easy to wear, can sleep with them in, symmetric cable. Great for speech, less good for music.
Cons: The thin treble undoes the benefits of an unboosted lower end and it ends up a bit dull.
goldenears.net has tested the CX 280 (same IEM but with inline volume control) and my CX 270 perform as described and expected. That is to say that they are one of the few budget IEMs which does not have exaggerated bass and is relatively well balanced overall. However that is not quite the same thing as being enjoyable to listen with.
I bought these mostly for listening to audiobooks with my player on a sleep timer. I think I have ruined a few expensive sets of earphones by crushing them under the weight of my fat head and kinking the cables at the ear and at the jack, so I decided I need something cheap, with symmetric cable, no silly booming bass, simple and durable, and small enough to sit in my ears without pressure. I had some RHA MA150 for this purpose but two pairs failed in a matter of weeks (one purchased, the other the warranty replacement) and I lost interest in that brand.
These Sennheiser CX 270 seem fine. For speech such as audio books and radio plays and talk radio they are ideal. For music I would slightly hesitate to recommend them because really deep bass is absent and the high frequencies are a bit thin. This is the kind of sound signature I would think ideal for Skype and similar so it's a bit odd that they are sold as a music IEM and without options for inline microphone and smart phone controls. My experience of genuine Sennheiser products is that they are durable (my HD 500s lasted over a decade, my CX 95 were bought in 2009 and now look scruffy but work perfectly, my Momentums just seem built to last forever) so I think if you have a particular use in mind and you can get past the marketing BS and into the specs and measurements you can probably find exactly what you need. I know Sennheiser are very boring for people who think the holy grail is designed and manufactured exclusively by some obscure workshop in Shenzen but sometimes what really matters is the damn things both sound good enough for the intended use and do not break or fail.
If you are thoroughly averse to silly boooming pseudo bass but only have £11 then these are a very good choice. Probably you can't find a well made, durable competitor at this price. If you can afford more then do so.