Since having Tyler I have a mental list of must-haves which have worked well for me and him, these include a baby swing, bouncer, jumparoo and of course a bumbo!

A bumbo if you have never seen one before looks slightly like a little arm chair with a raised bit to go inbetween the babies legs.
Many bumbos have belts, to keep the child secure. I believe they know offer a free "repair kit" to install belts into those that dont have one after many complaints where babies had fractured their skulls.
The one I had didn't have a belt but did have a tray which I always kept clipped on to ensure Tyler couldn't get out. As they get older they can arch their backs and get out so its worth keeping that in mind. Its also claimed that some babies are able to wriggle out.

A baby can use a bumbo once able to support their head properly. It helps them gain strength in their back and neck muscles and learn to sit up properly. It also gives them a new perspective of the room.

I used it at times when cooking, putting away laundry etc to have Tyer in the room with me, able to see me and in one place!
This isnt a necessary buy I will admit, you could use a bouncer or lay them down. They will learn to sit just fine on their own but i got a lot of use out of mine and Tyler loved it.

It is worth putting down some safety advice, though it seems obvious I didnt know all of this (for example: not putting it anywhere other than ground level) before attending a home safety course.
- Only use bumbo at ground level: A bumbo should not be put on kitchen counters, the table or any raised surface. Even though the baby is small some ARE able to move it slightly and if it was to fall this could be very dangerous for baby.
- Do not leave baby unattended in bumbo.
- Do not use bumbo near stairs.
- Do not use as a booster seat, high chair, car seat, bath seat or child carrier.
- Do not use bumbo near water.

I'm not sure why its not for use as a higchair. I admit before we purchased a highchair I did use this to feed tyler baby rice and his first foods. I think the reason may be because weaning isn't suggested for babies who can't sit up unassisted.

Bumbo suggests you only buy their seats rather than a cheap version (well to be fair, they would say that!) this is because the design of their seat is made in an ergonomic design which supports baby. I see no problem in buying this second hand (mine was second hand) or off a reputable site but I would reccomend buying the official Bumbo seat for peace of mind that your baby's posture is being correctly supported.

Bumbos (now sold with belts) are currently selling on Amazon for £27.72 and the play tray is going for £9.13. If you get as much use out of it as I did I think this is a reasonable price. I personally got mine second hand for a mere £15 (bumbo and tray).